The following are the symptoms of prostatitis:
Trouble passing urine or pain when passing urine
A burning or stinging feeling when passing urine
Strong, frequent urges to pass urine, even when there is only a small amount of urine to be passed
Chills and high fever
Low back pain or body aches
Pain low in the belly, groin, or behind the scrotum
Rectal pressure or pain
Urethral discharge with bowel movements
Genital and rectal throbbing
Sexual problems and loss of sex drive
Blocked urine
Painful ejaculation (sexual climax).
You don't have to complain of them all, but two or three together and you'll probably be bent over the table fairly quickly.
However, I wouldn't advise making an appointment just for these symptoms, unless of course, you are actually experiencing them, but if you're already scheduled for a check-up, then you won't be tying up a doctor and taking him/her away from folks who are really ill. If you just want the finger wave, find a playmate
Of course, as someone else has said, turning 50 will do it, as well as a family history of prostate problems will almost guarantee a DRE.