Thanks especially to Genghis and asmccuk.
Having thought about Genghis's suggestion of the gliding lift drag ratio I realised that that a fairly acccurate zero thrust would have to be set on the propellers otherwise you could be measuring some "disking" or alternatively residual thrust. Feathering both (all) is obviously the answer for the supremely confident !
Strain guages on the engine mountings would be my prefered option.