In reply to John Stainforth:
Easiest way is if they make a noise, once you're familiar with it (which I presume you are) then it's easy to distinguish from a crow's caw.
But I once came across a family of ravens in the lake District, the young one's voices hadn't broken yet, it was hilarious. Adults were gronking as usual but the young ones version was high pitched and squeaky.
When silent it can be more difficult depending on your view, ravens have a wedge/diamond shaped tail which is sometimes more apparent when in flight. Heavier beak, but on the ground, at a distance, outside "normal" areas for ravens, it can be difficult to tell.
Often you can identify by relative size with other nearby birds; easiest with jackdaws, crows (obviously) and buzzards (nearly as big whereas crows are obviously smaller).