A professional appraisal can be quite expensive. First, you should identify the pewter and see if it may be valuable. Early pewter was an alloy of tin and lead, quite dangerous for food or drink. Use older pewter for display only. Modern pewter is an allow of tin and copper or bismuth and antimony and is safe for food and drink.
Learn more about the pewter from the Pewter Society. There are several books about pewter including:
"Pewter: A Guide for Collectors" by Kenneth Ullyett
"Pocket Book of American Pewter : The Makers and Marks" by Celia Jacobs
There are also books available about British and European pewter.
If you think that your pewter is old, unique, or valuable you can find an appraiser in your area by checking out the American Society of Appraisers.