Everybody has their own crap-symptoms. For me, series of 3-sentence paragraphs are a good sign that I am writing around myself, or writing on auto-pilot. The use of the double-dashes is an instant flag, as well. There are other, stinkier things that crop up, too, like mellifluous passages that go nowhere or secretly dump info while looking beautiful. Those are just the obvious signs.
I'm looking forward to the day when I can catch my B.S. and whisk it away without breaking a sweat. I'm getting better and better about it, but I still have blisters from my pitchfork. :)