The capsule of a condenser microphone is actually a capacitor, with its two plates- diaphragm and backplate- very close together, and with an isolation resistance of several thousand million ohms (Gig Ω) between them. One of the plates is charged with a DC voltage. When the isolation resistance between the two plates is lowered, through enough dirt and/or moisture on the capsule's surface, the voltage gets a path to creep over to the other plate, its capacitance breaks down (or "backs down") electrically, and can no longer hold the charge. That stops the capsule from working properly.
You can test the health of your capsule by getting up close to the mic and breathe onto the capsule area. Humidity-heavy breathing will add enough moisture to an already contaminated capsule that the moisture will bridge between the dirt particles and form a conductive path. If the path is conductive enough, it will then allow leakage of electricity from one capacitor plate to the other: the capacitor collapses and the capsule's performance is affected.
You can hear when that happens, and do not need any test instruments: a mic with a contaminated capsule thunders, or produces whistling discharge sounds, or, in severe cases, the sound disappears for a few seconds or longer.
Usually, the original, high isolation resistance between the two plates can be restored with proper cleaning methods that also leave the super-thin gold layer on top of the diaphragm undamaged. This layer is often damaged with unprofessional cleaning methods like mechanical brushing.
