When skimmers are inserted, they often block the highlighting mechanism on the terminal. If you notice that it’s dim, this could be as sign of a skimmer.
Most terminals have a function that lights up the buttons which is something you may not have noticed before.
The same goes for the green light at the top of the device. If this light is not on, then it may be blocked by a skimmer.
Here is the placement and look of a functioning green LED light. This should turn on when the terminal scans a card.