The least expensive way is to use a vinometer. A couple of drops are added to the funnel end and then allowed to run through the opposite end. The unit is then inverted and then a line can be seen running down a scale. Whereever the line stops, a reading is taken relative to the scale. We have one model that sells for under $5 and another that sells for around $20. The more expensive one is easier to read and can also do sweet wines. Here is a link
http://www.thegrape.net/browse.cfm/4,9781.htm
The other inexpensive way is to use a wine hydrometer but you need a reading before ferementation begins and another after fermentation. By the wording of your post I am guessing you don't have a specific gravity reading before fermentation. If you did, you could calculate alcohol content by volume thus:
Starting Specific Gravity - Ending Specific Gravity
------------------------------------------------------------- = alc by volume
7.5
john