If an oscilloscope is not available, one can do the following:
Adjust the drive so that, in "tune", the transmitter produces 25W (not 100W) of output. Turn the transmitter off. Remove the Pure Signal coaxial jumper. Connect the Pure Signal output port of the internal coupler directly to the RX2 input (no additional terminations). Set the RX2 step attenuator to 30dB (to prevent overloading of the RX2 preamplifier and/or the RX2 ADC, and for additional protection of the RX2 components that follow the step attenuator). De-select the "ground RX2 on transmit" option. Now, with the transmitter output power, in "tune", set to 25W (not 100W)... place the transmitter in "tune" mode. Observe the RX2 panadapter reading (with RX2 tuned to the transmit frequency). If the reading is +6dBm - 6dB = 0dBm* on the RX2 Panadapter, then there is no additional post-jumper attenuation in the Pure Signal feedback path between the internal coupler and the RX1 step attenuator input.
*I.e. 25W is 6dB less than 100W.