This is a good question that all Dominants should ask themselves from time to time. You need to make sure everything you do is consensual. This means checking every time you play, and throughout the scene. Just because a sub enjoys something one time doesn’t mean they will another time.
Remember there are 2 sides to the coin, that it is a relationship and that you and the submissive have an equal input into everything you do. Dominance has your submissive’s enjoyment and pleasure at heart, if you’re forgetting that at any time then it becomes abuse.
Everyone makes mistakes. Another way to tell a dominant from an abuser is in their ability to accept they made a mistake and to atone for it. If you make a mistake, apologise and make sure that it never happens again. Don’t pretend it didn’t happen or worst still, make the submissive feel like it was their fault. That is abuse.