Take the sample for SG before adding the sugar. The hydrometer measures the amount of sugar in solution. After adding the priming sugar your SG would be higher than when the yeast finished eating the sugars in the wort.
This will give you the specific gravity at the time of bottling. If the yeast continue feeding in the bottle, as in the case where final gravity was not achieved in the fermentor, pressures will increase beyond the level of carbonation you wanted. If you notice excessive carbonation pressure check the SG again with a sample of flat beer from a bottle. If it is the same as at bottling time you're okay. You just have an over carbonated beer. If the SG is a couple of points lower than at bottling time you will need to take some measures to release the excess pressure to prevent the possibility of exploding bottles.
edit: Your SG samples should be at the same temperature for an accurate SG comparison/reading. Ideally the same temperature as the calibration temperature of your hydrometer. Often the calibration temperature is 60°F. There are charts to use for conversions when the sample is not at the calibration temperature.