We are traveling by TGV from Paris to Avignon and Bordeaux to Paris in April. (With a bit of driving in between.)
How do we...
a) determine if the trains have an upper deck?
b) reserve seats in that area?
Assuming you are planning to purchase those tickets in advance, I believe you will be given the opportunity to choose upper or lower (if the train has upper level seats). It's been 6 years since I made that trip but I had the option when purchasing the tickets on this website. I can't speak for any other ticket purchase websites (Seat61.com, Trainline.com, RailEurope.com,etc).
IF it is a duplex tgv you will be given a seat map like the one on linked from the seat61 website. https://uk.voyages-sncf.eu/sites/uk.voyages-sncf.eu/files/Seat%20map%20-%20TGV%20Duplex%20(TGD)%20(generation%203%20«euro%20duplex»).pdf
Basse means lower and haute means upper when selecting your seats.
Most of the Bordeaux to Paris trains are still single deck.