Here's what I did to discover a cracked wheel on an old 1991 autocross Celica ...

I took the whole wheel/tire, aired it up, and dunked it in my swimming pool which was closed for the winter, I then tracked down where the little bubbles were coming from. Guess what? It wasn't the tire, it was in the center if the wheel opposite of the tread. They were fairly low profile tires, so I assumed that a rock or something got driven over enough to push the tread up enough to crack the aluminum wheel. Porsche had big problems like this with their magnesium wheels, low profile and wide tires.

If you don't have a large body of water, just use a bucket or large cup. Put a liberal amount of dishwashing soap in it, fill it up with water, and pour liberally all over the wheel and tire after airing it up fully. You will be able to see where the bubbles and foam magically appear!