It's heat proof or fire proof. And I want to make sure, Bronzor's and Bronzong's are so annoying :P
Get a Pokemon with Trace. It will copy their ability.
Porygon, Porygon 2, Porygon Z, Ralts, Kirlia, and Gardevior work.
in a double battle get a ally to use E-quake, if it hits it has heatproof,if not, then levetate
I know the answer to this from experience if a ground type move hits it then it has heat proof(before hidden abilities) or heavy metal(once hidden abilities were added) if it doesn't then it has levitate.