Chestnuts are trees with a very distinctive leaf made of big lobes. There are two kinds of chestnut tree, and hey are different species. The SWEET chestnut contains nuts you can roast by a fire and eat. The HORSE chestnut or buckeye is a seed that is mildly poisonous (and much harder).
It is the horse chestnut that makes the hard seeds that some people use to play the game of conkers. That is because they are round. Edible chestnuts are NEVER round, but slightly dish-shaped, and you can oush a thumbnail into them.
Edible chestnuts are in a case that is more furry than a horse chestnut/buckeye case. The pictures below show this. The lower picture also shows the way a sweet chestnut can be displayed to make it more attractive to look at.