How to tell the difference if a fire alarm is being pulled just for a fire drill or when something is really happening?

i usually get told about a fire drill in advance..

The difference between the fire alarm being pulled in a building versus the fire bell being activated by the Fire Department for the purpose of conducting a drill, is that if the fire alarm is actually pulled, ALL magnetic door locks in the building automatically UNLOCK, all emergency lighting overhead each door illuminates, all elevators automatically return to the ground floor and become disabled without a Fire Department key and an automatic 911 signal is sent directly to all regional Fire Departments which bypasses the usual 911 Dispatcher who therefore is not the one who activates a call to the Fire Department.

That is why the interior of any building can be accessed from the underground parking garage by simply pulling any fire alarm located in the underground parking and the otherwise locked building entry door will suddenly unlock and let you in.

Is there a strange burning sensation on your skin, and does it smell like bacon frying?

Fire or lack of tends to give it away.