Researching the artist and the artwork online, in auction house catalogs and through online auction sites is the most direct way to determine value, as is getting an appraisal of the piece. The value of a signed, limited edition print can vary widely depending upon the artist, the condition of the piece and the number of prints produced.
If a site exists that still sells work by the artist, that is a good place to start. If you believe the print to be worth a substantial amount, you may wish to contact a professional gallery or art appraiser. If there is any question as to the piece's authenticity, this can also be a good way to go.
For an artist or print not likely to attract the attention of a Sotheby's or other high-end auction house, you may find eBay and other online auction sites with the same or similar works. Looking at both asking prices and actual prices paid for prints sold recently should give a good estimate as to the monetary value of the work.