Pop the cover off next to where the battery goes. Look through the slots there and see if the board underneath is black or green. If it is green Rev A and if black Rev B. The B works a little better than the A for some people but they are basically the same. I fail to see how it could be urgent to know which you have. If replacing it for some reason do yourself a favor and get the m8n instead because it works loads better than A or B.
I'm guessing you want to buy one that someone is selling? Not worth it. Buy mRo's made for solo M8N GPS. It is loads better than both the A & B 3DR GPS. mRo GPS u-Blox Neo-M8N / 3DR SOLO Upgrade
I also suggest you buy this RFI shield for the GPS. Frankly it's the best money you'll spend on the Solo. I have an old Rev A GPS and this shield gives it new life. Solo GPS Shield V2 - 3DR
Well its because the nearest civilization to me is 150 miles away and I was going into town today and I needed to know before I left. I was exchanging my drone because the live streaming is all messed up. So I just thought 'why not get the rev b while I'm at at it?'
See if there is a 'B' in the serial number printed on the UPC on the bottom of the box. If there is a B, then it will have a Rev B. If there are two 'A', then it's a toss up as I have seen some with Rev A and B.