Today i've had 3 moles cosmetically removed, they were burnt off. how do i know these moles were not melanoma or could be melanoma in the future?

You don't: Basically you can't know what a mole that has been destroyed was; a biopsy is the only sure way. But it they don't come back you can be pretty sure they were not cancer. If they do come back seeing a dematologist or surgeon for a biopsy is the next step. By the way if they do come back it does not necessarily mean melanoma, so don't panic, just get the biopsy done.