We can't use cell phone or anything - my old Detroit Diesel is too loud!
If I am the least bit in doubt I get out and walk it myself. Once we start the evolution there are two, and ONLY two, hand signals: "mon back" (typical "come on" arm movements) and "STOP!" (criss-cross arm movements). At all times she has to be in my vision or I stop.
The rule is that I am ultimately responsible, but if she is IN ANY WAY uneasy or unsure as to what is going on she is to signal STOP! and then we confer or usually I get out and look.
The point of having two and only two hand signals is that there is no confusion, they are easily differentiated, and they can be seen even if the light is getting low.