Against an unknown at 50NL, I think you can do this simply as an exercise of combinatorics and look at hands that he might think are strong enough to do this with:
75s (closing the action, this is hardly too far-fetched a hand to consider)
A6, 76, 86, 65, 33, 44, 66, 99, 64, 96, and then a bunch of flushes.
There are of course more hands he could be playing, but a good starting point is the strongest part of his range to see if we can beat even that 33% of the time (which is all we need given his pot sized river bet). So PokerStove tells
us that versus a range of...
99,66,44-33,AdKd,AdQd,AdTd,Ad8d,Ad7d,A6s,KdQd,KdTd,Kd8d,Kd7 d,QdTd,Qd9d,Qd8d,Qd7d,Td8d,Td7d,96s,86s,75s+,A6o,9 6o
... you're 40% to win and have an easy call.
And that's the STRONG part of his range. If you put in some overpairs and even some bluffs, you're way over the top.