As said, all the ones I've looked at have the same slots cut out in the metal shield, but have a translucent sheet, which you should still be able to tell if its rev3/5 or rev6/8... Although I'm not sure that helps much 
hmm, yeah the plastic looks reflective so unless you shine a light on it you don't see anything & even then it's tricky to make out. It would be interesting to get pictures of each revision though as it might be that it's enough, if there are slight variances in component placement.
I'm not sure why you'd want to know what revision it was though. What people want to know is:
Kickstart
Agnus
Denise
You can probably figure out which revision it is from that tbh.
I think rev 3 was kickstart 1.2, rev 5 was kickstart 1.3 (although some people seem to have rev 5 with 1.2 roms, so whether you have a commodore or amiga key might give it away). Both were OCS.
Rev 6 was ECS but still kickstart 1.3, it might only be ECS agnus though and not ECS denise.
Rev 8 will definitively be ECS, probably 2.04 but it's been known to be 1.3 as well. I don't know if there is a way to tell the difference between a rev 8 configured as an a500 and a rev 6 (possibly only if you can detect the difference between a 1mb agnus with 0.5mb ram and a 2mb agnus with 0.5mb ram).
It wouldn't surprise me if commodore shipped some rev 3 with 1.3 though if they found some old pcb's in a box somewhere, the same for keyboards. You got what you got.
