Best thing to do is always read the manufacturers data plate on the A/C compressor before looking into the electrical panel.  That way you'll always know the breaker size required.  The nameplate will look something like this:

The Minimum Circuit Ampacity figure will indicate the wire gauge required, in the above it would require 10 gauge. It will also list the Maximum Overcurrent Protection device. This means that your panel breaker cannot be of a greater value than what's listed. This is regardless of the ampacity of the branch circuit.

This is a great topic Mike and one that can be very misleading since sizing circuits for electric moters is a bit different than residential branch circuits.  Perhaps this link will explain the sizing in more detail for anyone wanting to know a bit more.

Hmmm, for some reason the above post didn't attach my name to it.  Oh well, It's DJ Skelding from Safeguard Inspection Services just in case ya'll wanted to know.

I see 12 gauge wire multi tapped onto the main lugs. NO BREAKER. Supplies the garage power. "It's been that way for years and never had a problem" Ever hear that before on your inspection??? What an answer.

Sometimes it may be ok, but its best to flag it and recommend further evaluation by an electrician.