Merlins have only a faint whisker mark. Merlins do not hover as kestrels. It's always difficult deciding on the size of a lone bird flying in the sky. Thus, Merlins may easily be confused with the larger Peregrine Falcon. Peregrines also have races which mimic the coloration of Merlins - especially the dark coastal races.
Peregrines show a strong solid helmet which comes down to form a pointed cheek patch.
Merlins have 3 or 4 wide dark bands across the tail. Peregrine Falcons have more and thinner barring, making the tail less conspicuously banded.