Just a few questions.

if your periods are irregular there could be some underlining problems that could keep you from ovulating so you might want to talk to your doctor about that. but to answer your question you can try to track your cycles to see if you ovulate and when. there are ways to do it by taking your temperature and signs to look for. or you can buy some OPK's (ovulation test's) it's like a pregnancy test but you pee on it so many day's after your period and it lets you know if your ovulating. the box should have instructions as to when to have sex and when to text for pregnancy. There are websites to help you learn to track it with the temperatures and other signs one website is called "fertility friend" also I joined a website when I was TTC and found lots of other woman in the same boat and they all had a lot of help. that website is http://www.thebabycorner.com/boards/
if you have any questions feel free to pm me also!

Without charting, all you can do is take a test 2 - 2 1/2 weeks after you have unprotected sex. The best thing to do, though, is chart -- read Taking Charge of Your Fertility by Toni Weschler and check out www.fertilityfriend.com for the basics. That will tell you for sure when/if you ovulate each cycle and thus when you can expect your period to come (or hope that it doesn't) and when you can test.