How can i tell if/when i ovulate?

I had my son feb 2010 and got the depo shot at 6 weeks after having him. I got it 2 more times after. I was due for my last shot december 2010 but DID NOT get it. I have only had 2 kind of periods, idk if that's even what id call them, since I stopped the shot. I read on a website like webmd or one similar that you can still ovelate and not have a regular period every month. How can I tell when I ovulate with things like cm and position of cervix? I started taking vitex a couple weeks ago to try to help regulate or induce periods but no results yet, but I read it can take 2-3 months for vitex to have an effect. My husband and I are really wanting a second baby but it seems that me getting the depo shot after having my son really screwed me up! I had been on depo for 4 years before got off it november 2008 and was pregnant by may-ish! Not this time though...