For me it's about building rapport with a buyer first, establishing a relationship where they will be honest with me. I find it's several steps to get to the truth. Before showing a buyer the first home, I "qualify" how they determined that a particular home is in the "price range." Their answers guide me to determine if they have owned before or not, and if they have been using "online" mortgage calculators. If they are not already pre-approved, I wait until I've shown them 2-3 homes. That way, I have shown good faith, that I want to work with them, and am trusting of them to be honest with me about being "ready, willing, and able" to purchase.
However, before I take them for more showings, after the first few, I meet with the buyers at my office to discuss "hiring me" by presenting the Buyer's Rep Agreement. I personally ask for a small amount of money upfront (this also gains commitment for cash buyers), that will be a credit to them at closing. At that time, I establish a date by which they will make full loan application as part of our agreement to work together. If they are unwilling to do that, or don't do it by the agreed date, it allows me the opportunity to determine how much of my time they will receive, if any.