by experience and knowing your fossils and ammonites well. so for instance you find a such a such ammonite in a certain bed then you know it bed number such and such then you have to think ah so the bed i am looking for is up there or further down. then you have to find an area where it comes down to beach level or chest height to look at it.
PLEASE BE AWARE HOWEVER THAT DIGGING IN CLIFFS IS STRICTLY FORBIDDEN AT MOST SITES. Including dorset. Best way to get around this is view the rock type in the cliff that you are looking for and then see if you can find fallen blocks on the beach to study. ONLY COLLECT FROM FALLEN MATERIAL.