Instructions
Look for a maker's mark. On Fenton Hobnail, the mark is on the bottom of the stopper, not on the glass itself. All Fenton Hobnail pieces came with a stopper, even items that don't normally come with stoppers. Early models used wooden stoppers, later models used matching glass.
Check the bottom of the glass for a ground pontil mark. The pontil is the scar where the glass was attached to a glassmaker's rod. Fenton hobnail glass was all handmade. Machine-made glass does not have a pontil mark.
Refer to published guides or catalogs. There are many guides for Fenton glass. If the pattern does not appear in any of the guides, it probably wasn't made by Fenton.
Consult with other collectors or antiques experts. It may be worthwhile to hire an antiques expert to identify the glass for you. They may charge a fee for their services.