I use the following and it seems to work well, but there might be a better solution.

if(String(obj) == "[object Bitmap]")

Trace out the object type using trace(obj) and then replace Bitmap with it. I use custom classes so one of my objects was a Note and i would use the following:

if(String(obj) == "[object Note]")

Basically just a string comparison, i didn't really want to research any solutions so i came up with this for myself.