Remember that if you're on the pill, you aren't ovulating, at all. Personally, i only use the pill and i've never become pregnant, and my boyfriend always finishes inside me. You can always monitor yourself and find out which days you're 'supposed' to ovulate, but again, you aren't ovulating on the pill so using any kind of tempature montiors won't help you out.
If you're still worried about pregnancy you're just going to have to continue to use a condom or something like that. But if you're using the pill 100% correctly, i think you can let loose