I Recently had this issue and I noticed the rounder head between the two and the lesser having the more rounded. also the greater is a tad bigger. It also depends on were you live and what might be more common in that area.

Lesser Scaup, I saw some of them here in Costa Rica.

This is definitely a lesser scaup. The only other bird it could possibly be is a ring necked duck, but notice the head shape and the absence of a white ring around the bill. Additionally, the white in your picture extends over the top of the Ducks back, whereas the ring necked duck only has white on the sides. This means it is definitely a lesser scaup. Glad I could help! Also, greater scaups are a rarity in Texas at this time.