Seems pretty decent to me, and Seasonic is a known reputable brand. (That doesn't guarantee they've never manufactured a dud, though.)
Wouldn't even power this one up on a dare.
Definitely a cost-cut PSU. Probably not dangerously bad (at least they invested in heatsinks) but seems to have omitted some of the AC stage, so I would avoid it.
Doesn't look bad at all.
I'm always leary of Guy-In-A-Basement brand names like this, but I don't see anything that looks obviously wrong with this one. The cables exit the side of the enclosure, which may not work in some tower configurations. If you own a Dremel, that problem is easily solved, though.
Obviously cost-cut, but doesn't look TOO bad. The important stuff is there.
Now, about how you would tell... This can be tricky, and it's really as much a gut feeling you get when you know a little bit about PSU design. Bear in mind, there are two factors that contribute equally to a good PSU: Good design, and good components. Cost-cutting can compromise either or both of those factors.
Let's talk about design first.
A typical switch-mode PSU will have a few stages, beginning with the AC section. Generally, the topology goes something like: Bring in the mains, go through a fuse, exit out to the front panel switch, arrive back in the PSU enclosure and go through a filtering stage (to prevent junk coming in and going out of the PSU -- the switching PSU topology is necessarily noisy by nature), rectify the AC to DC, and finally store the rectified bumpy DC in a couple of bulk capacitors.
Someone mentioned a distaste for the discrete diodes in the AC rectifier, vs. a bridge rectifier component. Personally, I don't really care -- it does the same thing. The only thing that matters is whether the diodes (discrete or single component) can dissipate heat well enough at their rated current. I would guess the discrete diodes are probably 5A parts, based on the somewhat obscured picture. That should be quite adequate, but maybe not as "heavy duty" as the case claims. 😉 The chunky bridge rectifiers on the other PSUs are usually elevated off the PCB, so they have more clearance for airflow which helps dissipate heat. Whether it's necessary or not depends on how much current is flowing through them, which is determined by the (designed) load. So... meh.
AC filtering is a sign of good design. It will work fine without, but if you don't see a boxy film capacitor across the mains, and/or disc caps from each leg to ground, and a choke (could be a toroidal coil, or a block that looks like a transformer) to filter common-mode noise, it's spewing junk into your house wiring, and that means it wasn't very well engineered. Move on to something better.
If there isn't a fuse between the mains input and the case switch, it isn't safe to use. Throw it away.
The mains caps take the brunt of abuse and are very important to the viability of the supply. Ideally they would be name brand (but often aren't), rated at LEAST 200V each (they're usually stacked in series across the mains, so the rating across both is really 400V), and have a capacity around 470uF. Numbers significantly less (e.g. 220uF) imply weak reserve capacity. Ideally, ALL capacitors, but particularly the mains caps, will be placed away from sources of heat like heatsinks and the bridge rectifier. There isn't a ton of room in most supplies, so just "not touching" sometimes is the bar you have to set. Minimum temperature rating is 85C, but 105C is better. (It's not just ambient temperature that matters -- it's also internal working temperature under load, and the higher the temp rating, the longer it will last at lower temperatures.)
OK -- next stage. The HV DC is chopped by a transistor -- which should be on a heat sink -- and fed through the largest transformer in the chassis. This converts the HV DC to high-current, low-voltage DC. From here, most PSUs will either have multiple transformer taps that provide the 12V, 5V, -12V, -5V rails, or some might be derived from others through additional switching stages. (In that case, you might see things on the label like "Max combined 3.3V and 5V = 150W" or similar.)
Small, light, janky-looking transformers are a sign of trouble. Tiny, pitiful heatsinks mean it won't be able to maintain stable operation under load, or it will just up and fail catastrophically. I don't even see the switching transistors on the YoungYear PSU. I'm not sure how that one even works.
Again, look for QUALITY name-brand capacitors on the output side, preferably with a 105C rating. The caps SHOULD be spaced away from heatsinks as much as possible. Size is dependent on a few factors, such as rated load, whether they're using multiple parallel caps per rail, the switching frequency of the DC-to-DC stage, etc. 2200 to 3300uF is a good baseline. If you see one 2200uF cap on the 5V rail, and a sticker claiming 15A, it's probably junk.
The output stage should have some coil inductors -- either toroidal or the kind that looks like a vertical post with thick wire wrapped around it. Ideally at least one per rail. This is a filter inductor, and helps make sure the output DC is smooth and relatively noise-free. It can be done with JUST a capacitor, but it will have considerably more ripple.
The final thing is overall sense of care in the design and parts choice. There should be some monitoring circuitry somewhere. It could be discrete components on the PCB (look for ICs near the output), surface mount stuff on the solder-side of the PCB, or a separate little card mounted vertically on the PCB (e.g., Soletek). This is typically the brains of the PSU, and will watch for things like over-voltage, under-voltage, over-current, excessive ripple, no-load conditions, etc. This is important because, if the PSU fails, it should shut itself down safely without taking your downstream load with it. The PSUs with barely any components on the board are engineered to do the bare minimum required to provide power. Not good.
Look also at cable gauge. Thin, scrappy wires imply an emphasis on cost. They don't need to be monsters, but they should feel adequate. If they're too thin, at the best case they'll drop some voltage as heat along the wire. At worst, they'll get hot under load and be prone to insulation failure under a fault condition -- particularly if the over-current protection is under-designed.
Now, at this point in the lifetime of all these PSUs, I consider re-capping a necessary maintenance procedure. The input filter caps tend to fail in a spectacular way after so many years, and the electrolytic (bulk, round cylindrical) caps aren't likely to meet their specifications anymore either. In some cases, they will leak corrosive electrolyte on the PCB, or may be bulging at the top. Even if they look OK, the internal resistance may be increasing, which generates more heat, leading to failure, and contributes to excessive output ripple. Replacing these parts is relatively simple, but it does help to know a thing or two about capacitor design to pick suitable replacements. I also replace the fan as a matter of course.
If you're testing these supplies on a bench, keep in mind they don't like to operate without a load. I've got some Delta supplies I recently re-capped, and they really wanted to be attached to a computer. Just having a HDD and optical drive attached, the regulation was terrible (11.2V on the 12V rail, and 5.3V on the 5V rail). One of them went into protection mode with just the HDD plugged in. That's actually a good thing -- if it hadn't, it might've destroyed itself or the HDD. A good solution is to use some automotive incandescent bulbs, or similar hefty load, to give the PSU something tangible to power. Load resistors are OK too, but you really should pull a couple amps, which will create some heat. Get those high-wattage sand-cast resistors and keep them in free-space, or at least off of paper and skin! 😀 Create a Molex test-harness that puts some load on both +12V and +5V. An old broken motherboard might work as well, as long as it's actually pulling current. I don't really like testing on good components, because they might turn into bad components if the supply is unhappy.
Hope this short novel is helpful. 😉