I'm sure your LHBS will have a proper test jar/tube; mine isthe typical plastic one, works like a charm.
Balrog makes an excellent point...make sure your sample is devoid of bubbles, spin the hydrometer in the tube, and that it does not stick to the sides. Spin it 3-4 times, take your readings, at least 2 of them will be the same (usually). If they're all coming up different, even by a few points, do it again...On bottling day for the Fat Tire clone we had a couple good readings at 1.012, then got busy with prepping for bottling...came back to the sample jar later and we had 1.010. That degas step in the sample tube is important...just sayin.
Another idea: you're in the carboy now, right? Use the light on your phone, or a bright flashlight, and hold it on the outside of the carboy near the bottom. Watch for yeast still floating around, watch for small (or not so small) bubbles rising off the bottom. I usually ferment in buckets, and do not go to secondary unless I'm dry hopping or something...so I don't get to see this phenomena...but I do have a cream ale in a carboy right now, I can send you pics of 3 weeks in after a very slow start (for whatever reason...it just didn't take off like I thought it should). The short story here is you can now see where you are, what's going on, etc, and judge your actions accordingly.
A good indication your beer is done when your airlock has had zero activity for several days, AND there are no significant bubbles on top of/center of the beer, AND there are a limited number of floating yeast rafts (usually small anyway) floating on top. Sanitize your new thief, take a sample and see if you've hit FG.
My opinion...others with more experience may have better advice, and, as always, YMMV.
Cheers, mate. You made beer. It will be fine. And keep us posted!