Red wire in ceiling box: traveler or smoke alarm interconnect?
I was able to come up with two approaches 


This requires a 9V battery, a wire-leads 9V battery snap (like one you'd find in an old transistor radio or such), and your smoke alarms to have battery backup.


First, turn off the breaker.  Then, take a 9V battery snap and wire it red on snap to red wire and black on snap to white wire, then snap a 9V battery in.  If it's the interconnect wire, your smoke detectors should start going off like crazy as soon as you snap the battery in.  If it's a traveller, or your smoke alarms don't have a battery backup, nothing will happen.  Just remember to disconnect the battery snap before turning the breaker back on!


This requires a multimeter and that your smoke alarms have a TEST button that sets off the whole shebang, or some of that "canned smoke" used for smoke alarm testing.


Have a helper trigger the detectors with either the TEST button or the canned smoke while you measure the DC voltage from red to white -- if it's the interconnect wire, you should see 9V DC there while the alarms are going off.