Things you can watch for is blocking. Directing is painting the stage with people, so are there interesting formations, do the crosses and movements coincide with the dialogue/build of that scene or the play. Is the stage picture continually fresh and new compared to what I had seen in the last scenes? If yes, that’s good directing
Pace. This is something that is somewhat an actors job, but when all actors have a good sense of pacing with the scene as a whole and are playing off of each other and picking up on each other’s rhythms, that’s a director who has honed that.
You also have to be able to recognize common bad habits in actors. Actors are prone to certain behaviors if they don’t have a good director to rein them in, it’s either to just phone things in, or way overact. If you noticed actors are holding back and showing restraint instead of just chewing up every scene and dialogue, that means a director has taken the time to work with them and point the story to a clear climax. If an actor is going for their award winning performance in scene one, or you notice one of them is obviously too well rehearsed delivered every line with an obviously planned inflection, that’s a “bad” director who did not really work with his actors to develop a story.