Ok my little brother stuck maxell tapes in my GL2, I always use sony premiums. I did a test on a sony premium and it looks like the heads are ruined (big square blotches randomly flash sometimes)

Is this fixable?

Get a head cleaning tape and use it as directed on the tape's instructions. Repeat once if necessary. This should get you back in shape.

If not, you'll have to send the camera to Canon for head cleaning.

What's the best way to know they it is clean for sure? If I run a half hour recording and no blotches pop up should I be in the safe zone?

Is it ok to use any cleaning tape? and then go back to sony premiums or will that cause a chemical reaction between the cleaner and the tape?

A Sony cleaner should do.

How would you know? You'll know. :-))

http://www.dvfreak.com/tape.htm