villain is 38/0 only 16 hands

Full Tilt No-Limit Hold'em, $0.25 BB (5 handed) - Full-Tilt Converter Tool from FlopTurnRiver.com

MP ($21.49)
Button ($6.26)
SB ($26.58)
BB ($38.24)
Hero (UTG) ($25.67)

Preflop: Hero is UTG with K, K
Hero bets $1, 2 folds, SB calls $0.90, 1 fold

Flop: ($2.25) 4, J, 7 (2 players)
SB checks, Hero bets $1.75, SB calls $1.75

Turn: ($5.75) 5 (2 players)
SB checks, Hero bets $4, SB calls $4

River: ($13.75) Q (2 players)
SB bets $19.83 (All-In), Hero ?

I would fold, opponet may have hit back door flush (least likely), 2 bad pair, or most likely trips, 20$ not worth calling for a 13.75$ pot

It's not a 13.75 pot, it's a 32.67 pot. ~$19 to win ~$33

You have to call at least sometimes in these spots or you're going to get bluffed to death. Is this that time? idk but I'd have trouble laying it down. If I'm beat I make a note and play the next hand. Seeing an unknown's cards is often worth something.

This is a marginal spot and calling and folding both have merits. The key question here is how he has played post flop in previous hands. If he doesn't raise PF but is aggressive post flop then this is a call. If he is loose-passive this is a fold. 16 hands isn't nearly a large enough sample size to derive any sort of actual "read" from but I would request time then quickly check the recent hand history and attempt to infer how he played from the previous hands.

A push on the river is often times the nuts from fish. This is an instant call against a maniac though.

I think this is a fold too many weird hands got there like QJ, AJcc, KJcc, JTcc. Sometimes this is like AQhh, QThh that rivered TP but he certainly can take this line with QJ and sometimes sets (not always but they will show up some non-zero amount of the time to lower our river equity). Sample size is small but this guy isn't going to converge to a TAG, so he is most likely a loose passive and river shoves from loose-passives aren't usually bluffs. This is a generalization but readless I think its a safe one to go by as a default.

The question is, is it worth $19 to find out which one he is.

It's very read dependent OP. Difficult to say just reading the HH.

With such a small sample I think I'd have to fold it here. There are too many hands that beat us, with only have less than 7 invested in this hand, this is a "pick your battle" moment. Is folding exploitable? Sure it is. . . but he's only seen YOU for 16 hands as well.

Do you really want to pay off some aggro monkey that got lucky calling OOP with Q7? (not that I think that's what he has mind you)

To make money at poker sometimes you have to fold the best hand and wait for the right time.

I called and he showed AcTc which made me very unhappy.

And now you know he'll chase backdoor draws (which we all already suspected). Value bet him to death in the future. No normal sized bets either. Pot sized at least. He doesn't care about pot odds, so pound on him.

Villain is hardly bluffing here. Think about it this way. When he calls two streets he's either got a marginal hand, some kind of draw, or a monster. He never has complete air. Now we have to think about his range that overbets here - its going to be the draw that got there, air, and possible the monsters that aren't really monsters anymore. All others check and pray for a showdown. 7x certainly isn't being turned into a bluff here.

He never c/c 2 streets oop with air so that part of his shoving range is gone. What's left? The draw that got there, the misplayed sets that he's pounding at the wrong time, but very very rarely a hand you actually beat. Just fold man.

Fold because you have no history on the opponent, so just assuming he's an average NL25 player you're good here like 10% of the time