River seems like a fold in hindsight, although its very tough to guess if they are barreling air here or not, how do you determine your call frequency?

partypoker - $0.10 NL FAST (6 max) - Holdem - 6 players
Hand converted by PokerTracker 4: http://www.pokertracker.com

BTN: 100.5 BB
SB: 116.7 BB (VPIP: 18.59, PFR: 15.25, 3Bet Preflop: 9.08, Hands: 3,175)
Hero (BB): 100 BB
UTG: 174.1 BB (VPIP: 25.79, PFR: 19.94, 3Bet Preflop: 11.90, Hands: 680)
MP: 267 BB
CO: 94.8 BB (VPIP: 20.35, PFR: 15.05, 3Bet Preflop: 6.07, Hands: 1,210)

SB posts SB 0.5 BB, Hero posts BB 1 BB

Pre Flop: (pot: 1.5 BB) Hero has 6h Ah
fold, fold, fold, BTN raises to 2.5 BB, fold, Hero calls 1.5 BB

Flop : (5.5 BB, 2 players) 8d 5c 5s
Hero checks, BTN bets 3 BB, Hero calls 3 BB

Turn : (11.5 BB, 2 players) 6d
Hero checks, BTN checks

River : (11.5 BB, 2 players) Kd
Hero checks, BTN bets 6.5 BB, Hero calls 6.5 BB