How to test an unmarked capacitor?

Assuming that "fluke meter" refers to a multimeter/vom (volt-ohm-milliameter), the answer is no. You need a capacitance meter to measure uf. And I doubt that it is practical to test for the voltage rating by any means. You would have to raise the applied voltage to the point where leakage current started rising excessively, which could very conceivably inflict permanent damage.

Regards,
-- Al

My Fluke has a capacitance function built in. But it will not work in circuit. There are devices that will.

Nearly all Flukes will measure capacitance with surprisingly good accuracy (especially the new 1xx series). You can measure the value in the circuit in only limited circumstances - but to avoid a lot of EE blather, the short answer is you need to have it disconnected from the circuit.

However, you cannot measure its voltage breakdown. I can guess at electrolytics just by looking because I am super familiar with them but I would not advise you to attempt it. A mistake would be very costly to your equipment.

Arthur