The only way to be sure it is not getting hot is to take off the side and feel the power supply inside the case IMO.
Does Your Power supply have more than one fan? I have seen up to three in some power supplies. take a Kleenex tissue and drape it over the fan and see if it moves in a breeze.
Also is it possible that the power supply has a thermal controlled fan? I have seen many a power supply with bad fans and they all got hot after a while. The danger in that is a hot power supply could send weird voltages out as it heats and possibly damage other parts of the computer.
I have also changed many a fan in a power supply. They are usually 12 volts and you just match the fan size, splice the red wire to the red wire and the black wire to the black wire and use heat shrink tubing to insulate the wire.