I was working at a client a few years back. Lets call this client Futz Inc. Futz was building a very complex application that was getting ready to go into the construction phase. Just as the construction phase began, the final version of the specification was sent to the copy center(this was before email was mainstream).
The cover letter of the specification asked the folks to review the specification and get back with any comments.
Well, the copy center made a mistake. Because this was a large document, they printed the specification duplex, or a page on each side of the paper. They accidentally skipped every other page of the specification.
So, the specification was obviously incorrect with every other page missing. Well, no one got back with comments. So, no one read the specification.
Since then, I use a trick. If I believe my customer is not reading my documents. I place an obvious error in the specification. For example I may say "There is one square wheel on the wheelbarrow".
If the customer says "Scott, we should not have the wheel square" I say, "Oh what was I thinking, of course it should be round, sorry".
But, if the customer does not ask me about the square wheel, I know to be very careful. I schedule meetings with the customer where i walk through the system in detail and ask questions.
So, if you want to see if your customers are reading, your stuff, make an obvious mistake and see for yourself.