Once you've seen ravens a fair few times - and as they are getting rather more common again one does see them fairly frequently if you go out walking in hills and craggy places - (and even less wild places I've even occasionally seen them here in the Chilterns, and I saw them on Salisbury Plain last time I was there) - you get used to their way of flying, which is different from the way crows and rooks fly. It's like spotting green woodpeckers by their swooping flight which is quite distinctive and I immediately know a GrWd as soon as I see it fly, often accompanied by its distinctive warning call. If ravens croak that is also distinctive - I often know there's a raven about as soon as I hear it. Same with choughs. If you are unsure if you have seen a chough, you haven't, as their way of flying is such you couldn't mistake them for any other crow.

So when you are out with an experienced birdwatcher in some place and he points to some raptor far away in the sky and he immediately and confidently tells you its an X, when it seems far too far away to possibly make out any of the distinguishing marks in the bird book, that's the reason - he's seen it so often it is immediately evident to him what it is from its habit.

So, in sum, to start with you'll have to rely on getting a good enough sighting to make out the tail shape and other distinguishing features, but you get used to them and then they become obvious.