How do historians know that the Harappans grew wheat and barley?

Clues to the type of food grown or caught in any ancient society are usually found through a combination of historical study, logical estimation, and archeological evidence.
If a civilization has permanent structures or advanced housing and city buildings, a historian will begin with the assumption that this culture was agrarian or lived off crops that could be grown and animals that could be raised (as opposed to nomadic hunter-gatherer societies that would need to be continually on the move in order to find their food source and hunt it down). Permanent buildings signify a permanent food source that can be controlled (e.g. crops).
Next, by identifying dwelling places, granaries, and other store houses, or even the local dumping grounds, archaeologists can identify what was part of daily life and local diet. They may look for the bones of animals or fruit pits—anything that will last over the centuries. In the case of the Harappan people or the civilizations of the Indus valley, archaeologists found wheat remnants in the store houses used to process the grain for further nutritional use (such as flour). For further information, archaeologists will also examine the bones and teeth of any human remains to understand more about the diet of the local people.
Here you’ll find some pictures of Mahenjo Daro, an excavation site responsible for much of the information we know about the Harappan Civilization.
https://www.bbc.co.uk/bitesize/topics/zxn3r82/articles/zghy34j

https://www.harappa.com/