You'll have to open up your computer to put the graphics cards in so why not open it to check on the power supply? I'm sorry, but there is no way to check it without physically looking at it or getting the model number from someone who has seen it. There are programs that will monitor the stability of the 5V, 3.3V and 12V rails but nothing that will give you all the specs you need.

However...

Do not merely check the power supply's wattage rating. There are 500W PSU's that cannot support high end graphics cards. You need to open it up, get the manufacturer and model number, then google that to find the specs. You need to look for the number of amps on the 12V rail. If it has multiple, there's a good chance it will, then you can roughly add them together to get the total. This needs to be above 30 amps minimum, preferably above 36 if you're going to overclock. That's just for one card. For two you need to shoot for above 40 minimum.

Also, it will not support two ATI or Nvidia cards, it's one or the other. Nothing out there supports both Crossfire and SLI, only one or the other. That is determined by your chipset. If you have an AMD or Intel chipset then you can Crossfire. If you have an Nvidia chipset then you can SLI.