If your PS2 has 8 screw covers, it will be a version 4 or Higher.  On the back of the PS2 console, there is a black label, as shown below

Now, just find your model number from the following list

Version 4:

SCPH-3000,  SCPH-30002,  SCPH-30003,  SCPH-30004,  SCPH-35003,  SCPH-35004

Version 5:

SCPH-30000R, SCPH-30001R, SCPH-30002R, SCPH-30003R, SCPH-30004R, SCPH-30006R

Version 7:

SCPH-39000, SCPH-39001, SCPH-39002, SCPH-39003, SCPH-39004, SCPH-39006

Version 9/10:

SCPH-50000 SCPH-50001 SCPH-50002 SCPH-50003 SCPH-50004 SCPH-50006 SCPH-50007