Air bubbles in canned tomatoes
Octern,

It's a realative thing.  What you're trying to determine is: where these gas bubbles generated out of something inside liquid portion of the can?

The reason that can be hard to determine is that many cans have a little air trapped in them.  If the can has been agitated at all (doesn't need to be extensively), then you can get what look like bubbles coming from the tomatoes themselves, and it can be pretty hard to tell.  

I generally look at it from a quantity/location standpoint: are the bubbles throughout the liquid, or are there only a few along one edge?  In the end though, with tomatoes, I'd do what SAJ said.  While they're probably OK, the penalty if they're not is pretty darned severe, and canned tomatoes are cheap.