Being that your periods are irregular, you woud probably only know you're pregnant by taking a test. The longer you wait to take the test, the stronger your pregnancy hormone is, so waiting makes the test more accurate. There is a test that is made to test 5 day before your period is due - by First Response - so it's more sensitive, but it's also less accurate.
You could go to a Pregnancy Center and get a FREE test - look one up near you at this site: