Insert your middle finger, up to the second knuckle and you'll probably find it straight away. If you've put your finger in any further than the third knuckle, that's probably too far. So, somewhere between 2nd and 3rd knuckle in. It's just behind the base of your penis, roughly. It's not very easy to find your own, so try someone else's. It's very easy to find if your finger is already in someone's arse when they cum, as the prostate will tense up, and go really hard, like a muscle. I'm not sure if it is a muscle, but it's the prostate tensing up which squeezes the spunk out.