what kind of things would you see?
Depends; sometimes I'd see the yeast wouldn't settle out, as in not at all. Sometimes the yeast and/or the beer would have wierd colors, or odd-looking bits of debris, odd stuff floating on the surface, etc. Any of those would be signs of suspect yeast that I don't want to repitch.
These days, when I harvest yeast, I see yeast/liquid separation starting within minutes; by the next day, the yeast is usually settling out nicely.