Depends on what you need I guess.

Old supplies need to be tested on the 5V section because that is where the most load will be.

New supplies need to be tested on the 12V section for the same reason.

If you test a new supply using a 150W load on 5V it will always fail unless you have a 1000W+ beast (most general supplies seem to be 120W max on the combined 5/3.3V).

I use a generic tester on standard ATX supplies (some old OEM supplies are wired differently). Newer testers show actual voltages on an LCD screen (plan to get one sooner or later). You can connect an old power hungry HD drives that doesn't work properly for a load while the tester is on.

Any newer supply should shut down if voltage is too high/low else you should not be using it to begin with. Even the old crap AT supplies will shut down if you have a dead short somewhere.

Caps go bad from heat, use, and bad design especially during the Athlon XP and P4 era, deal with it.

Some Socket A boards like Gigabyte GA-7VA utilized 5V channel for CPU power stabilizer. Athlons and Durons were also hungry like P4, but boards making much load on 5V instead of 12V. So S462 is a good platform to test PSUs.

But the only scientifically-correct way is to make real current on all channels like listed on label. It may be do like stress tests of diesel locomotive generators. 😀 By using HUGE resistors and water cooling (bathtub-size boilers).

There is this guy called Falco75 and he is a kind of celebrity here in Italy. Mostly known for his contribution in the Italian forum of TomsHardware.
He also does youtube: https://www.youtube.com/channel/UCg8YjbZgHqv784ZVXu7y8nw

He manages to check about every available powersupply on the market and gets to test them in different aspects.
Voltage, ripple, noise levels, components used inside, circuitry ...etc.
In his youtube channel you can see he built a "box" with what I think are Halogen Lamps (not sure?) and he uses them to put load on the PSU voltage rails (3.3, 5, 12V).
I think he once wanted to make available the components inside his "magic box" but I don't know... I will check and will update if I find more info. 😀

Yep, light bulbs are a great load for testing. If you can still get resistive elements anyway...

I suppose you could rig something up using car headlight bulbs. At least for 12v rails.

I'm wondering if you can use a automotive battery load tester to test a power supply most of them do 12v/6v.