The Firex Smokes will tell you which one is going off.
This smoke alarm features an LED indicator which can be seen through the Push-to-Test button. The LED indicates the following:
Blinks once a minute — indicating normal operation.
Blinks once a second — smoke alarm senses smoke and simultaneously sounds an audible alarm.
Blinks once approximately every 10 seconds — False Alarm Control™ (FAC) feature activated (model GC only).
OFF — another smoke alarm in the (Interconnected system only): network has sensed smoke and is signalling this alarm.
See Page 5 of: http://www.icca.invensys.com/manuals/firex/110-427G.pdf
[This message has been edited by Bill Addiss (edited 01-15-2005).]