You bring people on stage to assist with something not to be the eyes for everyone else.
Actually,
People are brought onto stage very often so that the rest of the audience can see their reaction and build off that. It is somewhat of a way to get the reactions starting.
Depending on the size of the stage, how many people, budget, etc. You could, if you really needed to, get a cam set up to a screen behind you. Also getting a few people up on stage (one or two; someone who you know will have a big reaction to their reactions prior) to "help" you. Let them check out the string, etc. But when they react strongly, so will the rest of the audience, even if they cannot see, but take sight into consideration, you want everyone to see.
Keenan