How can I tell if I have a prostate cancer?

Rectal exam and PSA. Digital rectal exam and serum psa levels are what doctors usually use to determine the next steps in care. Rectal exam showing one or more hard, irregular nodules on the posterior surface on the prostate is considered positive. Serum psa over 4 ng/nl is also seen but it is nonspecific, and even values <4 can possibly be in cancer cases if rectal exam is positive. That's why biopsy is done.
Most prostate cancer. Is found by a pattern of rising psa's; a digitally abnormal prostate exam is uncommon, and most are found at biopsy with rising psa. Psa driven diagnosis began about 1990. Symptoms of bladder outlet obstruction (frequency, urgency, dribbling) are not specific to prostate cancer, but bladder outlet obstruction from benign cause. There is risk of making a diagnosis you do not eed to know about.
DR can help. It is difficult on your own to decide this. Certainly being male and over the age of 50 naturally increased your risk. However, your doctor can review your symptoms, if any and decide if there are certain tests that can help determine your risk.