Here are two other ways:
1: Using qctool: running
qctool -g myfile.bgen  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.)