You pick up a book or check the "look inside" feature on Amazon, and read the first page.
Do you want to continue reading? Enough to buy the book?
On the Writer Unboxed site, there's a feature called "Flog a Pro: would you pay to turn the first page of this bestseller?"
As the title suggests, the feature reveals the first page of a popular book, asks you to decide whether you'd pay to continue reading, and explains why they would or wouldn't. You can vote and see how others voted.
I opted for "no", as did 54% of the total voters--but since this was a new post, only 17 people had voted.
The post's author, novelist Ray Rhamey, also voted no.
Like Rhamey, I felt that too much of the first page was devoted to the unremarkable musings of a character I don't know yet. The book is part of a character series, so that might not bother people who have read others in the collection.
Focusing the the first page is an enjoyable way to get yourself to be analytical about what works (or doesn't) to grab the reader, a good source of lessons for our own writing. It's not only browsers who pay attention to the first page, so do agents and publishers. Crafting a strong first page is equally important for screenplays.
However, avoid the temptation to try to make the first page perfect before you write the second and third. That's better left for the second and subsequent drafts, if you want to avoid driving yourself crazy.
Would you like writing advice from the classic and modern greats? You'll find it in my book, Your Creative Writing Masterclass, published by Nicholas Brealey/Hachette, and available from Amazon or your other favorite bookseller.