You dont HAVE to open the case to get a pretty good idea of Version. by taking the memory cover off, you can see 1) if the bios is gooped, 2) the switchpack 4 and the voltage switchpack above the memory slot.
Turning the things on might tell you if it is a 2001 (always V5?).
So, looking in the memory door with a flashlight:
No switchpacks, BIOS gooped... V3
No switchpacks, no goop... V1, V2
switchpacks, no goop, no security torx holding the stand to the case: V4
switchpacks, no goop, security torx holding the stand to the case, cant get into BIOS/2001: V5
None of this tells you if any of the IDE pin headers have been clipped, or any of the other variations of the IO.