Your periods are a very different thing than the bleeding that occurs in the absence of birth control - it's totally normal to have a period that is heavier/lighter or longer/shorter than what you have while on birth control! A period is completely different medically than what amounts to a withdrawal from birth control, which is why most doctors say you can safely skip 'periods' while on birth control by imply not taking the week of placebos.
The only way you can be sure you are ovulating is to track your temperatures - the ladies have given you great links to show you how to do that. If you don't feel like doing that, if you are getting a monthly period, you can be pretty sure that you are ovulating at some point during that time. To figure out when, you can use ovulation prediction kits, chart cervical mucus, and/or chart temperatures.