Set the audio gen to about 100kHz (sine wave) use full level and connect the output across the electro under test - then connect the scope probes direct to the same cap, not the generator - this is important.
You should see a small voltage at 100kHz on the scope - say 2 to 100mV rms.
By comparison with known good electros of similar ratings, one can note the residual voltages and determine if a given electro is OK.
If you put a 1 ohm test load on the audio gen - you can easily find the output current and then use the voltage readings on the scope to get actual ESR values for electros.
Tests can be done while caps are still in circuit - but be careful to make sure they are fully discharged first !!!
.... Phil