3 way circuit options to indicate if circuit is active
Using a Z-Wave, Insteon or similar type of "smart" switch would be the simplest way, as just requires replacing the two existing switches. Expect $50-70 per switch. The is the closet thing I'm aware of that's an off-the-shelf solution to your problem.



You could also run a new wire between the two switches, to provide a switched return line that could power an indicator light. I don't know what the situation is like if it's feasible to run a line or worth the effort. Quick search finds some "3-way pilot light" switches that would work. Just to be clear, these require 4 conductors between the two 3-way switches rather than the usual 3.

You could also use a 120V panel-mount indicator light. You'd need to expand to a 2-gang box and custom mount the indicator in a blank faceplate. 



Yet another option would be to build a current-sensing light circuit: basically, a current ring on the power line that is capable of sensing the light being on, and in turn, lights up a LED. This would require a custom circuit (though you can find lots of examples on the internet), a transformer to get the ~5VDC (or whatever you use for the circuit), and space to mount all of this stuff (you can't/shouldn't just mount a circuit board inside an enclosure that has 120V in it, it needs to be in a separate enclosure -- offhand I'm not sure if this is part of NEC, but if not it's just common sense).