Just kind of backing up what Zoltan has said here in regards to possibly finding yourself a viable test power supply. He gave you a good idea, but I'll take it one further, if you decide on using a test power supply, in some cases form factor does not apply. You need not to worry yourself so much if your test power supply is going to be a proper or perfect fit inside that computer case. Your primary goal is just to check for power, as in, will the computer turn on! If you find an old computer laying around you, which it's power supply is known to be good, remove it from that machine, even if it is an odd ball design or shape, or is to fat or skinny, that all means nothing. If it won't 100% fit, just take your computer, remove the now suspect power supply, make sure everything about it is removed, lay it on it's side, with the gut's of it pointing straight up, while jostling the power supply from hand to hand plug in all it's power connectors, as long as it's only going to a couple minutes or so, it's safe to test the power supply with it in your hand or balancing it on the computer case somehow or resting beside it on a table, counter, work bench ect. But if you go for the hand option, only a couple minutes tops, after that power supply's can sometimes start to really cook, you don't want a burnt hand!
Also power rating is kind of important here, just make sure the test power supply is within the range of the now old and possibly dead power supply.