hypothetical (!) answer;
they don't use thumbscrews, but odds are the account will come to light. all accounts are link to National Insurance numbers, and are cross-referenced with the benefits system. the DWP is constantly informed of accounts that attract over a certain amount of interest, where there is a benefit interest.
just to add to this hypothetical answer, the DWP gets a form with the full account details, and will then contact the bank quoting the given account numbers, obtain full statements for the benefit period, then probably prosecute the criminal involved.
let me know if you need further info, it's my specialist subject :o)