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The simpliest method is to measure with scope ripple before regulator at the charging capacitor, and to compare it to the ripple at the output. For example if you measure 3Vp-p on the cap after diode bridge, and 30mV at the output PSRR will be
PSRR = 20log dVout/dVin=- 20log(0.03/3)= 40dB