Here are two other ways:

1: Using qctool: running qctool -g myfile.bgen [1] prints out a summary of the input data, including the number of variants and samples in the file.

2: Using bgenix: E.g. the command bgenix -g myfile.bgen -incl-range 22:0- -list would list all variants on chromosome 22. (You might have to index the file with bgenix -g myfile.bgen -index first.)

Best, g.