Occasionally when the flop comes down players will look back at their hole cards. For example this may happen when the flop is monotone, ie the board has 3 cards to a flush on it.
This tell is generally an indication that the player is checking if one of their card have connected with the board because they remember their cards are different suits.
As a result this is usually a good indication that their cards are not suited as if they were suited they would remember and know they either have the flush or they have no chance of the flush. Either way they would not need to check their cards to see. This obviously leads to situations where good thinking players will check their cards in scenarios to trick other players on the table by doing it as a false tell. Although, this sort of thinking play does not occur too frequently.
Another possible reason for checking back to their cards is to figure out exactly what runner-runner possibilities they have.
Checking back at your cards after the flop is usually a sign of weakness. If you spot this tell on the table you should take the aggressive lead in the hand to take advantage of the weakness the opponent has shown.