Nowadays mostly done by a blood test which looks for the level of PSA(Prostate Specific Antigen) in the bloodstream.A high level of this antigen and the doctor may recommend a prostate biopsy.
A digital prostate exam may also be used and any symptoms such as difficulty passing urine,more frequent urination,more frequent night urination will be taken on board.
Starting to get checked at 50 is a good idea or earlier if there is a strong family history of prostate problems.