And, believe it or not, changes in barometric pressure can cause airlock activity in a completed fermentation. I've had suck-back, as well as bubbling, only due to a storm front moving in!
I think as you gain more experience, as SpanishCastleAle said, you can actually guestimate where you are in the fermentation better than you could without the experience. While I don't rely on airlock activity, I often can gauge about where I am. I confirm with a hydrometer, though. I can also use visual clues. As an example, I have a 7 day old AAA. It's been bubbling pretty slowly at 64 degrees. But I thought I'd check on it, and see if I could rack it. I pulled out the sample, and wow- is it cloudy! Full of suspended yeast and bubbly with co2. So, I knew that it wasn't at FG even before putting my hydrometer in. And, it was at 1.026, like I suspected. So, visual clues (dropping of krausen, cloudy wort), airlock activity, etc, all can be used by an experienced brewer without pulling out a hydrometer each time.
If the beer is crystal clear, without airlock activity, and it's been a couple of weeks, you can probably guess with reasonable certainty that it's done. But, confirm it with a hydrometer before bottling!
The other thing to keep in mind is something that is unique to those of us who keg. We can keg the beer, even if we're not at FG, if we like the beer where it is. We have no danger of bottle bombs, so we can be more relaxed. Of course if the beer is sweet and underattenuated, we'll let it sit longer. But if it's a couple gravity points higher than I'd like if I was bottling, I just shrug. It's easier to be relaxed when you're not concerned about underattenuation or bottle bombs!