Besides the polarity, the other caveat is that it should be a regulated supply, meaning that it will always output 13V regardless of load. Cheaper and older supplies frequently aren't regulated and their voltage starts out a lot higher than what's marked. The voltage then drops gradually in relation to how many amps are being drawn. That's okay for some devices but a laptop might not like the input voltage to deviate from what was intended.
If it's labeled as a "switching" supply then I believe it will inherently be a regulated type.
If the supply is OEM from another laptop then it's probably well regulated and quite safe to use.
If you have a multimeter then it's easy to just check the voltage at the plug. If it's regulated then it will be at the correct voltage without any load.