In hindsight, I would have asked my first question in the sandbox before asking it for real, because while I got a lot of answers, they were a bit disappointing.
I've used other Stack Exchange sites, but none of them have a sandbox.
There should be some sort of mention, for first-time users, before they submit the question, that the sandbox is available. Maybe something akin to the "Don't ask fluff questions" warning that beta users get before they can ask questions, or a popup like the "Don't use the word "I"" popup box.
Once the user has asked the question, it may be too late to suggest the sandbox.