How could I check whether the building area is available for the all buildings in the city which I want to analyze?
Welcome, for small area’s this looks something for Overpass.
While I do not understand what you are exactly looking for, zoom in on a small area with only a few buildings and then press the Wizard button
Then type “building=*” and run the query.
For a larger area overpass is not the way to go but you should likely get a OSM extract for the area you are interested in, import that into a database and use a database query to get the data you want, not sure how much experience you have with this but this is a start:
You obviously can’t check whether a dataset is complete by itself alone. The closest reference to OSM is Microsoft AI buildings (probably good enough for existence, while the shape is inaccurate), and some limited others in RapiD. Google Open Buildings is compatible too. Best to have the local government open data, probably at least Lidar DSM and DEM plus vegetation detection if no building polygons.