Usually most DMs ask the players, "Did you guys have fun?" Anything else, they may ask for feedback, but this is usually the most glossed-over question when in fact it's the most important to answer.
Players should be willing to say flat-out, "No." This gets the DM's attention and opens the conversation as to why. If he doesn't, or is not concerned, then that's indicative that he doesn't need to be DM anymore. Here is where you critique the DM, because it carries a lot more weight to say, "No, I didn't have fun because of X, Y, and Z." instead of saying "Yes, I had fun, but you did X, Y, and Z which could use improvement / changes / blah, blah, blah". The DM heard you at "Yes, I had fun" and then will wait for the Cliff Notes to come out on the rest of what you said. Star Trek comes on in an hour.
If my players tell me that they are not having fun in my game, it's my job to fix it and fix it quickly because the campaign's success will eventually ride on my willingness to respond to complaints or issues.
I had a DM a long time ago who was fairly imaginative, but there were two things about him that myself and the other players didn't like--1) he hated running any kind of combat and avoided dice rolling if he could help it. If we encountered goblins or whatever, he had them run away or call the fight in the our favor. 2) he railroaded us to the point that he stated what our character reactions were at times until he had to frequently correct him which only irked him.
At the beginning of the last session with him, we all had a sitdown with him and complained to him that we'd like some combats, get the dice rolling, etc. His reaction was to us that if we didn't like the game, we could always quit the game. We gave him one last session and kicked him out of the house after that.