How to check the amount of water available for the borewell in a rocky area? Area made up of graphite, borewell is 900ft deep

We have dug a borewell up to 900 feet recently, little water came out and the pipes are wet. Up to 60 feet casing has been done. The entire area is covered by graphite rock (according to the geo consultant). The borewell guy says, since rocks are present, even little water is enough and it will never dry up and also it is more than enough for a single house. How can we check the amount of water that is available? Please suggest.

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