Polarized capacitors have distinctive markings for +/- in some form or another somewhere. Are you sure that is a polarized capacitor?

If yes, try to measure its leakage current. A reversed polarized should have a higher leakage current. Can be measured with a DMM that has high input impedance (1G\$\Omega\$ or higher) for DC volts in series with the capacitor and a power supply.

Not all DMMs have the high impedance mode for DC volts. Expensive ones usually can be set for G\$\Omega\$ impedance at measuring DC volts (usually the high impedance is available only for mV DC range, up to a 1Vdc top, or so. Read the DMM usual manual if you are not sure about your DMM high impedance mode. If yours does high impedance DC volts, then use the DMM as a uA-meter by adding a DC source (or a battery) in series with the capacitor:

- set the DMM on mV DC, manual range, gigaohm impedance

- discharge the capacitor

- connect all 3 in series, the DNN, series with the capacitor under test, series with the battery

- read the mV DC on the DMM (the sign doesn't matter)

- reverse the capacitor and read again the mV (the sign doesn't matter)

- you should see fewer mV when C is correctly polarized

Think of the DMM as if it were a G\$\Omega\$ charging resistor for the capacitor. The "charging" current will be smaller when the capacitor is polarized correctly (less leakage) and higher when the capacitor is polarized in reverse (because of its higher leakage in revers), so the DMM will show many mV when C is incorrect placed, and fewer mV when C is correctly connected.