The stones are smoothed and polished, but they look very much like the stone you might see in a tumbling stream of water in the mountains. The answer is that they were found in the stomach cavity of a dinosaur along with many others. The environment in which the specimen has been found gives us confidence that it is in fact a gizzard stone. Suppose that we found the same stone by itself on a sand bar of the ocean. This problem has been very common in the sciences.
Eoliths are chipped flints.
They are rocks made up mostly of silicon that have a fracture that produces sharp edges. We know that ancient man used the conchoidal fracture of flint and chert to make arrow heads, spear heads, and knives. For many years, it was assumed that any time you found a eolith you had evidence of ancient man, and sometimes they were found in sediment that was very, very old. Later it was discovered that chips and scars on rocks can result from tumbling in a stream, and anthropologists are very careful today not to jump to conclusions about whether an eolith was intelligently produced or not. If the eolith were found on the head of a split stick with a cord wrapped around it to hold it and feathers on the end to make an arrow, it is doubtful that anyone would want to attribute it to chance.