It is remarkable how observant birds can be and also how accurate they can be with their identification skills. Many birds can spot a Sparrowhawk or Hobby in the air well before most birdwatchers. And they make this clear by letting other birds know by calling distinctive raptor warning calls. If you can get to know the warning trills of tits, for instance, or the alarm calls of hirundines, it can be one of the best ways of finding a bird of prey.
Hirundines also often lift up and flock together as they see a potential threat like a Hobby.
Similarly, larger predators flying are often ‘mobbed’ by other species. Listening for unusual calls from mobbing crows etc then looking up is a way many experienced birdwatchers first see a passing Osprey or kite, for instance.