A human player should be able to estimate the probability that a certain set of cards will win. This is implicit in the rule-based systems which give threshold hands for betting decisions.
There are two different measures for the goodness of a hand: the potential and the strength. Potential is the probability that the player's hand will become the likely winning hand (a 4 card flush counts for nothing but is very strong if a fifth suited card is dealt). It can easily be roughly estimated by humans - good players are usually able to estimate a hand's potential accurately, in terms of outs: ``the number of cards left in the deck that should produce the best hand" . In contrast, strength is the probability of currently being in the lead (would win if no further cards were dealt). This is often based on experience or knowledge of the statistical distribution of hand types, although knowledge of one's opponents is used by expert players to get a much more accurate estimate.
Knowing where one stands with respect to these measures is used to determine an appropriate strategy, such as raising to reduce the number of opponents, trying to scare one's opponents into folding by betting aggressively, and so on.