What you need to do is find out what type of motherboard you have and then find out if you have an agp slot or not. That is the key. If you have the motherboard manual, it should tell you in there. Once you find out if you have agp or not, then you need to find out if you have a x1, x2, or x4 agp. Once you know that, you can look up the specs of the card to see what type of agp slot it is compatible with.
It may sound a bit daunting but it really isn't.
Another way to tell is just take your case cover off and look at the slot your video card is in. If the slot is smaller than the other slots on your board, it is agp. If it is the same size, then it is pci.