In blackjack, the table composition has little impact because you're playing against the house, although noticing tourists who don't follow basic strategy can slow the game down and reveal frequent mistakes.
In poker, the table is everything: a table full of regulars will be tougher, while one with tourists offers more opportunities. Quick signs of weak play include players who limp a lot, make minimal or inconsistent bets, talk about "luck" more than ranges, and handle their chips haphazardly. In blackjack, the difference is marginal; in poker, it's decisive.