The answer is of course yes (that's why we put them on display). I don't actually mind answering the question, but any time I'm repeatedly asked the same question, I think there has got to be a better way to communicate the answer.
Signs are always the first option, but signs can go wrong quickly.
Then it struck me to use the same trick that restaurateurs and buskers use - you know when you see a tip jar with money already in it, you're more likely to put some in yourself versus a jar with nothing in it?
To translate this theory to book displays, we could start using dollar bills as bookmarks in display books, but I thought a better idea would be to always leave one of the display stands empty. It's subtle and non-verbal, but if someone sees that someone else has already checked out one of the books from the display, it might communicate to them that it's okay for them to check one out, too. Which is what we want them to know, especially if no staff person is around for them to ask.
I did this on all the displays around the Reference desk last week, and I'm waiting to see if anyone asks about checking out a display book. Usually it happens a couple times a week - so far so good.
What do other people do to let patrons know it's okay to check out display books?