They cross check lists of benefit claimants with bank accounts. If Mr X had 100K in a savings account and was claiming a means tested benefit he'd be called in and asked to explain why.
I think all accounts are now linked to National Insurance numbers, and NI number data gets cross-referenced. the benefit fraud people can sit back while capital references appear on their desks!
All payments must be payable to the third party or bank will not accept them, and of course you could cause problems for them. Their bank will want to know where the money came from and they would not have the paperwork to prove legitimacy.