how to differentiate RAM

Generally speaking, DDR and DDR2 can only be differentiated by the labeling, SD RAM looks decidedly different though:

DDR/DDR2 RAM:

SD RAM:

Also it wasn't around for that long but it was very popular for awhile so I think its important you know it:

Rambus or RDRAM:

oh thanks, by the way how to see AGP and PCI-Ejang_nara

AGP is usually offset of PCI by around an inch, PCI-E is offset by a couple of millimeters. In addition AGP is usually going to be less wide than a PCI-E slot for graphics cards.