I do get people who first meet me to ask to see properties in the first conversation, and I just say something like, "Wow, you're on top of it, go ahead and send me a copy of your pre-approval letter, and I'll contact the agent" Not confrontational, but shows an expectation, like it's the obvious step to be taken before seeing a property. Put the requirement on the listing agent and seller. I know if I were selling my home, I'd want only qualified buyers walking through-that's what open houses are for.
Another way I sometimes go about it is to ask what type of financing they're using. With so many FHA and VA loans being used these days, explain that not all properties qualify, and you need to be sure the property they're interested in does, and it's especially important to the seller and listing agent to know if they're going to take a home off of the market for a buyer. If they've got no clue, then, ask the usual questions, like how much they have for a down payment, to determine loan type. If they're vague, then refer them to a lender first. If they seem to have a handle on their finances, but haven't talked to a lender, and the property looks like it'll work, meeting them once can go a long way to establish rapport with a new client.
If a buyer is at the point of looking at homes, they should be at the point of writing offers. In fact, I use the phrase "seeing homes and writing offers" a lot when I first start talking with people, so they understand that it's one and the same. I pepper my conversations with what's required to write an offer as I talk with them and explain things during the initial search phase, so I'm sort of prepping them for it as they're looking online and asking questions. Then, when they're ready to look, they're ready to talk to lenders. It won't be a big surprise.
The way I look at it is, why would anyone expect me to take time out of my day to show properties, when they won't even take time out of ONE their days to talk to even one lender when it's THEIR purchase? A serious buyer will be ready to write an offer. If they're not ready, they're not ready to make visits.
It just depends on the caller.