Is there a way to tell if you were on birth control when you became pregnant?

You mean knowing whether you were still on birth control when falling pregnant? NO WAY. It is impossible to know so relax. If you work out when you conceived you can tellifyou were on birth control. However if you mean how do you know you are pregnant if you are using birth control, it is unlikely, it should rreally be called conception control, however if you think you may be, take a home test.