How to measure the efficiency of a full wave rectifier?
simulate this circuit &ndash; Schematic created using CircuitLab

From your answer I have understood you have a scope.
Put shunt resistor before the diode bridge (between source and bridge). Then with scope measure current trough it and also voltage over diode bridge. Product of those two is loss (Ploss).
Both these values are variable (AC) so you can multiply them on scope.

Because you already have Pdc, efficiency is Pdc/(Pdc+Ploss).