What are they doing when they check my fire alarm?

I would guess he had something like this: http://www.hoyles.com/acatalog/solo-detector-testers.html (about ¼ the way down the page)

The detector was probably a CO-type detector, so he had to actually saturate the detector with CO (sprayed inside the bowl) rather than just mist the detector. Ones that detect particulates can just be sprayed with an aerosol to test, but if it’s detecting a gas concentration you need a little more.

It’s a bit higher-quality test than just pushing the test button.