how can one tell

hi friend, i am glad to respond here, here is the information to help you to resolve this problem.I hope it is helpful to you. One, you'll need a motherboard that supports installing 4 GB of RAM on it, as well as a 64 bit processor. You can't run a 64-bit OS without a 64-bit processor, and many mobo manufacturer's will limit the amount of RAM that can be installed in it. Most of the newer ones max out at 4 or 8 GB, but some of the older ones won't support more than 2 or 3 GB. Two, you'll need to buy a 64-bit OS; XP Professional x64, Vista 64-bit, etc. Which is not cheap, in any sense of the word. XP Pro x64 will most likely cost you around $140, while Vista 64-bit will run you anywhere from $100 to $200 (depending on whether you got Home Basic, Home Premium, Ultimate, whether you get it retail or OEM, etc.). good luck to you.

I guess one way of knowing it is to look for the motherboard's model number then check it on the net. PCIe socket is not a guarantee that it is 64bit, it is hard to tract changes of these things lately due to rapid changes of chipset, processor and motherboard.