£50+5 at local Grosvenor. 6k guarantee spanked to £7,200 (partly thanks to hero's second bullet...). 20k starting chips. You join us in level six. Blinds 200/400. Hero has 19,500 and has only been at the table five minutes.
Two limps to my cut off (mid position kid flicks his 500 chip in, hijack fairly decent player for this level and has chips calls with confidence). I make it 1,500 with

.
Small blind is a creative and superloose player. He (30k) and HJ (35k) call.
Flop

. (Pot 5,200)
SB leads 1500, which is called. I make it 4k and both call.
Turn

. (Pot 17,200)
SB checks reasonably quickly but without any obvious adrenaline change. His head is resting on his left hand, leaning away from me and the chap to my right but with his legs obviously facing forwards (so he's twisting slightly). This has stayed his pose until this point. He released his cards to check with the same, right hand. FWIW he is almost certainly right handed.
We all check. HJ and I each take a little longer - around six to ten seconds - to check. I figure I can get all the bluffs to fire the river and not much value on the turn, as I don't believe I have a bigger ace against me.
River

. (17,200)
SB thinks for three seconds and bets 14k with his right hand again, now facing forwards with his left hand in a more normal position. (14k is exactly how much I have left). HJ folds quickly.
SB is Slavic or Russian, about 25 years old and is comfortable in a casino environment. He speaks fluent English, albeit with a heavy accent.
I want more information. I say "you must put me on Ace King" and he looks at the board immediately. He looks at it for a few seconds before looking back at me. He breaks the stare by checking his cards after a short pause.
I say "Oh no! I was going to call but then you did
that ! You can't
possibly be bluffing now!" He furrows his brow slightly and smiles on the left hand side of his face when the chap two to his left chortles.
I think this is such a tough spot, as I believe his primary range is 46/88/all clubs. He is capable of going a bit nuts with nothing, I suspect, but "going a bit nuts" also includes calling a chunk for a gutshot.
Trivial decision?