^^ You have to "test it" from time to time. But that doesn't mean hitting serves or bench pressing. That is (obviously) WAYYYY too much for your shoulder right now.
As I did the rehab I invented for myself, I could "feel" my shoulder getting better and stronger every week / month. But I didn't try anything as stressful as hitting a full serve or bench pressing until I was "pain free". And that took a long time.
But, I'm VERY risk averse, so I probably rehab / rest longer than is really necessary. I just don't want to re-tear anything and have to go through THE ENTIRE process again (and risk further damage).
"It is no measure of health to be well adjusted to a profoundly sick society.” ~ Jiddu Krishnamurti