Use a meter with a buzzer for continuity testing...
Each one has three terminals, a common terminal (C or COM), a normally open (NO) and a normally closed (NC). With no pressure on its button, check the continuity between C an NC - meter should buzz, press button it should stop.
Connect between C and NC, then press button, it should buzz only whilst pressed.
You can buy replacement microswitches for a few pence on ebay, if you can solder, but there are numerous sizes and types. Measure the ones you have.