If you find the tan can in the house and open it on the door there will be a wiring diagram which will have the model designation on it. That would be the easiest way. Here are a few examples that i can think of at the moment.
First alert 110 is like an Ademco 414o
First alert 148 is like an Ademco 15p
First alert 168 is like an Ademco 20p
First alert 162 is like an Ademco 20se
First alert 142 is like an Ademco 15se.