I've never done this, since I don't own a refractometer, but you might be able to come close, if the wine is dry. Read the final refractormeter value and use the excel spreadsheet backwards, as in it gives you a corrected Final SG based on knowing the beginning SG. So you should be able to come close to the beginning SG know your final SG.
to measure the abv before fermentation Use a brix refractometer. I have one and it will also help you to determine when your grapes are ready for harvest. Two things to know is your brix and the ph of your wort to get the most out of your wine or wash.
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.