Dropped frames

Hello

When I create an avi (I420) with my webcam, sometimes a frame gets dropped. 
When I read that file with libavifile later, is there a way of telling 
weither a frame was dropped or not? Currently I compare the two frames 
(memcmp), it works but it requires a lot of processing power I would rather 
be spending on something else :).

Thanks,
Hans.