Pewter is a distinctive metal, very different from silver or tin. It is an alloy of tin, copper, bismuth and sometimes antimony and lead. It may be mistaken for its close cousin Britannia, which is another tin alloy made with the same metals in different amounts, creating a harder, more durable substance than pewter. You can confirm that an antique metal object is pewter by feeling it and comparing it to other metals.
Compare the unknown metal to an object that you know is made of silver. Pewter is duller, darker and softer than silver. Clean up and polish both objects, then look at them side-by-side.
Check the unknown metal's surfaces in detail. Pewter is notorious for its softness. It is harder than tin but still easily scarred, dented and pitted by corrosives. If your object has noticeable pits, dents or deep scratches, it is more likely to be pewter.
Check for any age or maker marks. If you can establish that the object is newer than 1825, it is more likely to be Britannia, as this metal was introduced after that time. Maker marks also can tell you whether it came from a pewterer or a different type of metalsmith. Quality marks can indicate the grade of the metal as well.
Touch the surface of the object. This is the major "tell" for pewter. Although it looks smooth, it will feel slightly rough to the touch because of the multiple tiny pits and scratches it has accumulated with age. The surface also is very likely to be uneven.
Your object may be Britannia metal. Britannia is a harder, more durable and brighter tin alloy than pewter and was very popular in England and the United States for that reason. When polished, Britannia is brighter than pewter, although not as bright as silver. It also does not dent or scratch as easily. Use a general metal polish on pewter objects, not silver polish.