Identifying the polarity of the aluminium electrolytic capacitor
In some cases ( pun intended) the cathode lead is connected to the case.    

E-caps have a known reverse voltage breakdown effect that leads to thermal runaway and outgassing or detonation depending on the energy applied.  The reverse current is threshold dependent and above 10% of the forward rated voltage.  Therefore, you would not have reached that threshold with the DMM.

In order to test it requires looking at the DC voltage with a safe AC current applied that exceeds 40Vp.  Using impedance ratio, you may be able to compute this. A safe current limit would result in &lt; 1/4W worst case across the cap with > +/-50Vp.  The reverse polarity acts a weak zener diode threshold.  The large series R acts as a passive current limiter. The voltage applied is up to you and the power/voltage  rating of the series R.  ( final note in case of Murphy's Law, wear safety glasses)







simulate this circuit &ndash; Schematic created using CircuitLab