In reading poker tells, one of the most important things to do is watch your opponents look at the final river card. If it helps them, they’re very likely to only look for a short time. Then they’ll quickly place the card facedown and try to look uninterested, perhaps staring away from the approaching betting action. This is a ploy to make you think the card wasn’t a good one, and whenever you see this mannerism, you should be wary, and you should not bet or call with marginal hands.
Conversely, if the opponent keeps staring at the card longer than necessary, that’s an indication that it didn’t help. He’s trying to make you think that he’s interested in it. He isn’t, and you can bet medium-strong hands for profit.