A refractometer is more of a "field tool". It is used to measure the brix level in grapes in the field to determine when to harvest, among other variables. It can also be used to measure your must before yeast is added. However, a refractometer is not accurate once fermentation begins. The alcohol in the liquid will throw it off significantly and the equations for compensation are very complex and still not very accurate.
If you can get more of the same juice you started with, you can take the SG of that. That will give you a round about figure to start with. If not, like sour_grapes said, an ebulliometer (expensive, over $1000) or send it to a lab are options.