Yeah, airlock activity is exciting because it's something immediately tangible to the brewer, but as far as when the beer is done or not comes down to gravity readings.
I use a refractometer (this is the one I use
) to track the activity and once it stabilizes I'll use a hydrometer to get a true FG reading.
Refractometers are nice because you only need 3-4 drops of beer to take a sample reading but the alcohol content of the beer will throw the reading off. There are compensation calculators out there, but even then they tend to be inaccurate by several gravity points. So, like I said, they're nice to track a steady march to FG until the activity plateaus off, but a good hydrometer is really the best method of getting an accurate FG reading to record, though the downside is that the hydrometer uses a lot more beer. I find a combination of the two tools limits how much I lose to the "scientist's share."
To pull reading for the refractometer I use a 25mL glass Mohr pipet (like this one
) and for the hydrometer I use a 2oz glass turkey baster, both of which I sanitize in iodophor before taking a reading. The pipet is long enough that I can pull the sample from anywhere in the column of liquid in the fermenter, but it's unnecessary to fill it since you only need 3-4 drops of beer for a refractometer . The baster has the advantage of pulling a lot of beer quickly for the hydrometer test jar, limiting how much exposure the beer has to the outside world. As long as you follow sensible sanitation precautions there is little risk of infection.