Safely probing low voltage isolated DC/DC converter rails with an Oscilloscope
If you have access to a decent 4-channel Oscilloscope you can use a single-ended probe on each of +Vin, -Vin, +Vout, -Vout. (You can leave the gnd clips for each probe unconnected.)
Use your oscilloscope math function to subtract (+Vin - -Vin) to measure the input voltage and do likewise to measure the output voltage (+Vout - -Vout). In this way you are measuring all signals with respect to a somewhat arbitrary GND reference, but the point is it is the same reference point and therefore is canceled out by subtraction.