*sigh*
Ok, YOU'RE not opening it up.
Your installer will have to.
There are lists which show which PS2 version is which, simply from the numbers on the back. As for GAP vs. NO-GAP, the only way to tell that I know of is to look at the BIOS chip. The BIOS chip is easily identifiable. Look at the diagrams. Look at the ONE chip that's different. That chip is the BIOS. Now find it on your board...
As Bighap said, if you just print out all three diagrams your installer will be able to tell.
- Gurm