It's hard to avoid subvocalizing when you're thinking about avoiding it. It's like not thinking about the pink elephant.

I feel like that experiment would be easy to contaminate..

Sometimes if I'm stuck in speech/subvocalizing mode it can take awhile to get out of it.

I think the easiest way to tell if I'm subvocalizing or not by checking my current reading speed. Subvocalizing can slow things down.

Also I think I'm more liable to subvocalize during fiction, with lots of dialogue involved. Acting out the characters in my head slightly.