Collapsed borewell identification

How to identify a collapsed borewell?

Dear Sir,

The motor of a bore well which we dug around 290 feet in 2017 got stuck . The motor was removed and re-boring of 6 inch was done upto 350 feet . After the re-boring too, we feel that something is still stuck in the borewell, which is making it difficult to place the submersible pump in the bore at 80 feet. The total casing is upto 100 feet. How to check if the casing is blocked ?

Please suggest .

Thank you,