But the payment out will be on a bank statement, and if large enough, the executor should be checking what it was for and where it went. If the executor has done their job properly, they should have been tracing back from the bank accounts open at the time of death, backwards to previous bank accounts identified by transfers in/out. The money that opened every bank account open at the time of death will have been transferred in from somewhere, so that "somewhere" can be traced and that bank account reviewed too, and so on, until you get back the full 7 years, by which time you should have a pretty good picture of what money they had 7 years ago, the source of funds added (i.e. sales of assets, gifts in, wages/pensions in, etc), and the main funds paid out (i.e. living costs, gifts out, purchase of assets, etc etc). The vast majority of transactions will be nothing but "background noise", and it will normally be pretty quick and easy to pick out the big/unusual stuff that needs a bit of investigation. The hard things will be the big transfers between bank accounts, the accounts being opened and closed, etc., but like I say, you just ask the bank for the "half" of the transaction you can see and know about for details of the "other half", i.e. the bank account where the money went to/came from.
I've done loads of these, on a personal basis for both my parents and for my father in law and a grandparent, and for numerous clients on a paid basis, over the years. I once worked at an accountancy practice where we had a solicitor firm as clients who always "subcontracted" these jobs out to our practice, so we became quite adept at doing them. It's a cross between book-keeping, auditing and detective work. Doesn't take as long as you'd think it would as it's pretty easy to ignore/screen out all the irrelevancies and just identify and then find explanations for the big stuff that really matters. We'd get a junior to sift through the paperwork found in the house, such as bills in envelopes, old passbooks, cheque stubs, bank statements, investment reports, life insurance policy documents, guarantees for home repairs, old vehicle registration documents, etc. - once straightened out and organised into categories, very easy for someone more experienced to flick through and identify things that may actually matter. But like others have said, for the vast majority of estates it doesn't matter because the value of the estate is so far below the IHT threshold, it's pretty easy to satisfy yourself that there've been no "gifts" large enough to push it over the threshold.