I downloaded an app called Tuner Lite. It can hear my bowstring but I don't really know what it's telling me. It's also very sensitive. The room has to be dead quiet.
The tuner will tell you a note and how cents sharp or flat it is. There are 11 notes A, A#/Bb, B, C, C#/Dd, D, D#/Eb, E, F, G, G#/Ab. If you go left to right you'll ascend in pitch until you loop all the way back to A. The new A will be an octave higher and sound like a higher version of the lower note. If it's sharp that means it's a little higher in pitch than a true A, if it's flat it's likewise lower in pitch. I believe 60 cents in either direction is a new note.
It does have to be relatively quiet. It will pick up the pitch of any sound in the vicinity, but if you hold it real close, it should switch to the bow pitch when you pluck it for a while, then switch back to the ambient noise once the tone dies down in a second or so.