If you're REALLY confident it is fermented out you could put a small sample in a sanitized jar to be measured after you do get a hydrometer. You still risk bottle bombs if it's not done.
Or, I imagine you could rig your own "hydrometer" with a vessel that floats straight up in liquid (I can't think of the ideal thing, but possibly a used yeast vial with some pebbles in it to weight it a bit?). [NOTE: I'm completely talking out of my a** by the way but this should work]. Float that in water at 60F and mark where the water level is, that'll be your 1.000 mark. Then make up a solution with a really precise measurement of table or corn sugar to an exact volume so you can know the gravity of the solution (8.0 oz of table sugar with water bring to exactly 0.5 gallon would be 1.046). Float your "hydrometer" in that to get your 1.046 mark. Then float it in your beer, halfway between the 1.000 and 1.046 mark would be 1.023 and so on... I would think it'd get you pretty close.