Fish taking the fly do so with a variety of signs varying from Oh-so-subtle-that-we-never-knew-about-it type to the most-violently-imaginable-pull type which causes us to both lose balance and to wonder how such an aggressive take could not result in a successful hooking. As a general rule, however, the slower the retrieve and the more imitative the flies the more subtle the take.
With dry flies the whole thing is pretty obvious, after all you must be able to see a fish stick his mouth out of the water. At least you will if you are not totally asleep.
With nymphs, however, all you may know about it is a slight leader movement. Often not even that. Any strange leader or line movement must be hit with a strike.
Don't delay, you don't have the time. Try watching the water behind your flyline, where your flies are, silver flashes, glimpses of white (trout's mouths are white inside - getting hopeful here), a flattening of the water surface (in ripples), a slight water disturbance (in calms) all indicate that a fish has probably taken your fly.
Strike! You will never detect all takes, but if you rely on a pull you will detect very few. Constantly searching for takes is tiring. Take plenty of breaks to keep your concentration up.