The heating elements are glued to the back of the glass. Typically, the glass is poppped out and replaced. One way of checking it is to pop out both and swap them (don't mount them in the mirror housing, just plug in the heating element) and turn them on. You'll have to figure a way to see if one or the other is heating (infrared remote temperature gun, put an ice cube on the glass, leave them overnight to frost or fog up, etc. ).
You could also check the continuity of the heating element with a meter. If its not an open circuit on the bad one, then compare the resistance between the good one and bad one to see if there's a big difference.
-Jay