I think the correct terminology is "heavy" landing rather than "hard".
It's all really very subjective but I guess the safest course of action if you are unhappy about an "arrival" is to report it on the tech log so that licensed engineers can do a proper check. That way we all remain safe whether or not it was heavy.
In the course of a flying career you are almost bound to get a few touchdowns which are firmer than you would like - nothing to be really ashamed of if you are doing the job diligently.