I take it you are not a man, so here is what I can tell you since I own a prostate - they are farther in than most women realize at first. Depending on the position of the man you are exploring, it will be even farther in. If your guy is on his knees, plan on going inside his ass not less than 3 inches. If he is on his back, knees up, then you do not have to go in as far usually - perhaps a few inches. Use your index or middle finger, pad side facing the front of his body as his prostate in this ass-up on the knees position will be on the down side/belly side of his body. Just the opposite for the man on his back… prostate is then on the inside upper wall of his rectum.

It is easiest to find if he has an erection and you are playing with his cock and balls… the longer you have played without him coming, the easier it is to locate inside his body. That’s because the prostate will be enlarged and firm to the touch. And it will have a slight indented line down the middle of it, as if it has two halves on each side. IF you press firmly once you get inside his anus, pressing as you go towards the front of his body, you will easily find his prostate. And for fun, go beyond it to the top of the prostate, and finger around both sides, then go back to the top and start working your finger down the middle line, you will milk the prostate to the point where it flows out his cock!

Lastly… a guy will definitely know when you are rubbing his prostate, and so will you by the rhythmic and involuntary swelling and pulsing of his cock as you rub his prostate - it is exquisite! And undeniable. And if you milk him to an orgasm, with your finger inside him and on his prostate, the prostate will swell and get rock hard as he comes, like his cock… but hang on as his ass clenches so tight in rhythm to his ejaculation that he might push your finger out!

Now go find a prostate and massage it!