Non-destructive test to determine 110V electrical in backyard pipes?
Use the electric field, imbalanced, into your multimeter. Make a metal pickup, wrap halfway around (so you are likely to have imbalanced Efields, even if +-120 is inside.

Signal strength? Assume 1cm distance between wire and external metal pickup. Assume pickup is 4cm (wrapped halfway around) and 10cm long. Assume air is your dielectric. We'll use the parallel-plate model of capacitance.

C = Eo * Er * Area/Distance

C = 9e-12 * 1(air) * 4cm * 10cm/1cm 

C = 9e-12Farad/meter * 40 cm * (1meter/100cm)

C = 9e-12 * 40/100 = 9e-12 * 0.4 ~~ 4pF

We know Q = C * V, and differentiated with constant C, we get I = C * dV/dT

dV/dt = 377 radian/second[60Hz] * 120VACrms * 1.414 ~~~ 60,000 volts/second

I = C * dV/dT = 4e-12 * 60,000 = 240 e-12+3 = 240 e-9 = 0.24 uA.

Across a 10Meg Ohm DVM Rin, that 0.24uA produces 2.4 volts AC.

So, summary use a 4cm by 10cm piece of metal, to which you get a clean metal-contact (copper foil is good, whereas aluminum foil instantly oxides) with one wire of you meter. 

Now, you need "ground" of the other meter lead.