How do I tell if a PNG has a transparent background?

Hi All -

I've been using Lightroom from the start, but just ran into a new problem...I have a folder of PNGs that are all single images with white backgrounds.  Some of the backgrounds have been removed (in PS) and some are still white.  When I'm in LR, is it possible to tell the difference?  For example, can LR use a checkerboard behing the displayed photo instead of white, so the knockouts will be obvious?  Thanks!