P3A - how can I tell whether mine is Revision A or B and what's the difference ?

Look at the labels on the side.  Should say 322A or 322B (might be C). One of the difference of note is the transmission power.  It lessens with each newer craft.  The original 322 (no letter) had the highest transmission power for the radio. There are other changes as well as far as the body, motors, etc.  There are several threads here in the forum to search on.

Here's a link
http://www.dji.com/newsroom/news ... al-advanced-motors1