Open its mouth and you will find rows of sharp pointed teeth. These are much more pronounced in the trout than the salmon. The mouth can be opened wide to engulf quite large prey. The fish actually sucks in a large amount of water to catch food and filters it out using gill rakers at the back of the mouth. Prey species will depend on the size of the fish and availability.
Brown trout are particularly well known for their selectivity.
If there is a hatch of a certain fly on then you may well find that these are taken exclusively. Perhaps the best well known hatch is the Mayfly on the chalk streams of southern England. In the 1st and second weeks of June fish feed only on these flies. It is not just the species of fly which may be selected. Some brown trout will only seek out certain life stages.