As far as I know there is no standard method to indicate equal areas. The comments have already mentioned some good ideas, but to add one more, a simple (and maybe uncreative) method could be to label the interiors of each shape with a (probably capital) letter: Same letter, same area. If you already have lots of points and/or angles labeled with letters, you could perhaps surround an area-indicating letter with [brackets] to help differentiate it.
But whatever you do, be sure you tell your audience the method you're using beforehand. You should always do that whenever you introduce new notation.