depo shot

i’ve been off and on the depo for years. during the times i was on it, i had no period. so my question is now, i’ve been off the depo for some months now and my period has not returned. sooo how did you ladies just know you were pregnant without having a missed period to go by? & when do i know when to test since i don’t have period cycle to go by? any tips would be helpful. TIA!