For example, a defender who holds A532 or K532 will discard from that suit more readily than if he had held Q532 or J532. That will give you a clue in situations of this kind:

This is a side suit in a trump contract and declarer needs to establish one fast trick. In (1) East has made two early discards. Conclusion: he is more likely to hold Axxx than Qxxx. In (2) West makes an early discard. Conclusion: he is more likely to have discarded from Axx than from Qxx.

In (3) East makes two early discards. When you play the ace and eight he follows with the five and seven. Play him for Kxxxx rather than Jxxxx. In (4) West discards twice. He is more likely to have come down to Ax than to Jx; but if a low card to the king is headed by the ace, be inclined to play East for AJx.

Such inferences are especially strong when dummy has what may seem to a defender to be an establishable suit, as here:

West leads the three of clubs and South ruffs the third round. There is something to be said for leading a heart at once, putting West under some pressure if he holds the ace, but instead the declarer plays four rounds of trumps, discarding a diamond from dummy. (It is good play to keep the heart holding intact.) West throws a club and a diamond, East a club and a heart.

After cashing three diamonds South leads a heart and West plays the eight. South should finesse the jack. Why? Because of East's heart discard. With A9xx East, expecting the contract to depend on the heart guess, would not think it necessary to keep all four. But with Q9xx he would not let go a heart, in case declarer held Ax.