Country wines are easier to estimate because most table fruits have a starting gravity of about 1.050 (+/-) per gallon of juice and added sugar will increase gravity of the juice by 40 points per pound in each gallon.
Another way of measuring alcohol content is to measure the final SG and also take a reading from a sugar refractometer (usually available for around $30). It uses the fact that the refractive index of a liquid is influenced by the amount of sugar and alcohol present. The method was developed by makers of Port. The results can then be fed into the Home Wine Program to calculate the amount of alcohol and unfermented soluble solids. The program is available free from homewineprogram.com.