Quickest and cheapest way: observe your beer: krausen drops, beer begins to clear, temperature was raised to 70° after holding at your target temp for a few days and total time elapsed is now 2 weeks... you're probably good... probably.
If you want to be sure a hydrometer is the next option. Hydrometer readings will use up a lot of beer for a 1 gallon batch, especially if you start taking multiple readings with it.
More expensive is the refractometer. This won't give you an accurate FG reading, but it will tell you if the reading is moving (ie if its not moving your yeast is done). It uses mere drops instead of valuable ounces.
If you want an accurate FG reading, which is useful for a variety of reasons, you'll want a hydrometer reading. Again, the trade there is ounces of beer.