What's a safe method for determining the switching frequency of a blackbox ACDC converter?
simulate this circuit &ndash; Schematic created using CircuitLab

This is one way; with a load on the dc power supply output you can examine the ac ripple on a scope. Use a high-pass filter (HPF) that presents a load of 50 to 100 \$\Omega\$ (as shown in the schematic) with a 1:1 oscilloscope probe. You should expect to see 50mV or so.