On the back of the PS2 there is a SCPH ####### if it says this its a V7:
SCPH-39001
If it says SCPH-30001R its a V5 or V6....(Make sure there is a R at the end....)
There were a FEW V4's with a R but I doubt you will get one at a store now....
The back of the PS2 box may have this number also.... I'm not sure....