On one of my favorite rivers there is a spot where a pod of fish rise pretty consistently. Sometimes I've made the mistake of going after them with the same dry fly approach every time without actually really watching what they're doing. Several times I've been turned down by fish that were seeming to feed with abandon and wondered what was wrong.
Fish eating emergers will most often only show their backs, dorsal fins, and tails, and only occasionally a nose.
Fish eating adults will be a little more aggressive and you can usually see mouths if not more of their body, if they're eating caddis they will usually make really aggressive splashy rises.
Fish eating spinners, which are spent mayflies, will usually do so in slower water slicks and eddies and will just barely push their noses through the surface film, often making a distinct popping noise.
The other day when I finally watched close and saw that my pod of fish were eating mostly emergers I dropped a PMD humpback emerger about 8 inches off a dry fly and ended up catching a nice 20 incher and having a strike on almost every drift for a while.
Carefully watching rise forms and reacting accordingly will result in many more fish and less frustration.