K.P.
To answer your question, most tests have a "range of accuracy" if you look at the back of the box. Assuming a 28-32 day cycle you could assume that you were "fertile" two weeks after your period started and therefore would be able to conceive anywhere from day 12-18. Do you know when you had sex?
You should (in theory) be able to take a test 2-3 weeks after having sex and get a fairly accurate reading. You may want to take one each week for the next month to be sure (unless you get a positive reading).
Um... birth control is available in non-pill forms too. Just saying... pregnancy is preventable.