How are prostate diseases diagnosed? how do doctors know how severe it is?

Several ways.: Prostate diseases are diagnosed a number of different ways: by assessing patient symptoms and signs, by physical exam, by imagining such as ct, mri, or ultrasound, cystoscopy, or by biopsy. Assessing severity depends on the type of prostate disease: BPH severity depends on symptoms and cystoscopic findings, cancer depends on biopsy grade & clinical stage, infection depends on symptoms and imaging.