Most hormonal forms of birth control, like the birth control pills (oral contraceptives), and some IUDs recommend that you wait a few cycles before getting pregnant.
Methods of birth control like condoms, foam, diaphragms do not have any such recommendations on waiting. In fact, if you're just off a hormonal method and are trying to temporarily avoid pregnancy, these methods work very nicely for that brief period of time. (Read about how long it takes to get pregnant/)
If you are trying to determine if you are pregnant - think pregnancy test. A pregnancy test is the best way to tell if you are pregnant or not. Though you must wait until you miss your next period to get the most accurate results. This can be a home pregnancy test or a pregnancy test from your doctor, midwife or health department. In most cases you do not have to notify anyone of the test or the results.