Three days before departure, approximately, the MMB flight code changes from (for example) A320 to M20. You can then look up M20 and in that case you're probably safe. This time is when the aircraft is allocated to your flight, it's not precise, but is often in the lunch time area, give or take, and can be earlier if it falls on a Sunday. However the equipment can of course change right up to, and beyond, your advertised departure time. Not sure - on short flights - it's really worth monitoring, probably best to play safe rather than spend all day curtain checking.