Honestly, I don't really check the gravity before I crash cool it. Toward the end of fermentation, I just crank up the pressure 5 pounds or so a day. When I hit my carbonation pressure, I stop raising the pressure. I put my spunding valve output into a beer bottle with water in it. When it burps less then once every 4-5 seconds over a 24 hour period, the beer is done. Pretty crude method, but it works really well. I just did it with a Pils and after cold crashing I checked the gravity. It went from 1.050 to 1.009. Simple, stupid and cheap, just like me.
You will have to de-carb the beer sample before checking final gravity with a hydrometer (if you want to, or just taste it). You can just take a sample and pour it between two glasses until it stops foaming. If there is any carbonation in the beer, it will show a higher gravity than it really is. Or you can measure it with a refractometer and use a calculator to account for the alcohol present. The alcohol will cause the refractometer to show a higher gravity, the calculator will correct the reading.
Brew on!