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villian is unknown, just sat down a couple hands ago.

Party Poker 3/6 Hold'em (6 max, 6 handed) converter

Preflop: bottomset is MP with 7, 7.
1 fold, bottomset raises, CO calls, 1 fold, SB calls, 1 fold.

Flop: (7 SB) 5, 4, 6 (3 players)
SB checks, bottomset bets, CO calls, SB calls.

Turn: (5 BB) J (3 players)
SB checks, bottomset bets, CO folds, SB calls.

River: (7 BB) J (2 players)
SB checks, bottomset ?

is this an easy river bet against an unknown?
how about if the river was the J or J ?

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The interesting thing about this is that you have 77, which makes it significantly less likely that villain is on a str8 draw.

Let's break it down a little bit. Villain could have a pair or be on a str8 or flush draw. To figure out what villain has a read would be very helpful. Most players at higher limits would've raised with a pair on this board, which suggests that he's on a draw, which means that a river bet would NOT be correct. Since you have no read, though, and since you have 77, which makes it much less likely he's on a straight draw, the likelihood he has a pair and will call you with the worse hand probably makes the river bet +EV.