I am very happy to say I’ve been out of the game too long to know for certain about Richard Osman (I worked mostly with Americans and stopped ghostwriting in 2010), so I try to just enjoy his books. But judging by his public personality and the fact he’s been willing to appear on Taskmaster to support friends with almost no notice (because he lived near the filming location), I would say there’s a high chance that he was at least involved with his books, considering the fact that it would look pretty shite if someone who graduated from Warden Park School and then Cambridge wasn’t able to write a cozy mystery with just some heavy editorial support; he should have the chops and he’s talked about being a fan of classic written mysteries like Agatha Christie.
The best way to tell a celebrity didn’t write the book is to judge by what they normally do. Have they ever talked about books and reading, ever expressed any passion for the subject matter, have referenced writing a memoir for years before they actually do it, or are they just claiming the idea hit them like a lightning bolt? 🙄 Celebrity memoirs are almost always cash grabs, unless the celebrity was already a writer OR they really want to share something publicly, often before press manages to share it without their permission, and a full memoir is the best vehicle. There have been one or two female celebrities I know for certain who were very passionate about and involved with their memoirs though, and were willing to put in the collaborative hours necessary to get a good product, even if they didn’t have the writing ability.
Oh, the only English one I did hear about and I wasn’t directly involved, but rumour was that Danny Dyer DOES write most of the material in his books, but his “editor’s assistant” who had to compile it into non-random order of anecdotes and clean up the bluest parts was signed off for a short time for stress. That could absolutely be horseshit though (sounds like a story he’d tell about himself!!! The fact he hasn’t is what makes me think it might be BS); I wasn’t even in the UK at the time I heard that, and some “proper” male authors tend to be pretty judgmental and exceedingly bitchy over those they consider less capable. Interesting industry, but just like most others, a fuckload of toxicity and gossip.