It's all about asking the right questions.
Let's take an example of a trade booth at an Cookie Expo. Hundreds of people walking past every hour, many stopping and looking in... how do you filter who's really interested in buying your "Crunchy Chip Choco Bons" wholesale?
"Hi, are you interested in cookies?" - Bad question... why the hell are they at the Expo?
"Would you like to try one?" ... Bad question... you'll attract hungry people not wholesale buyers.
"Who do you work for? / What's your job description?" ... I bet you've heard this one 1000 times. boring silly questions.
When we do trade shows / expos - I make sure the "front line" staff have one question that they can ask everyone, and depending on the response we get to know how serious they are.
"What was the last wholesale order you placed?" (Okay a little direct, but for sake of this example I hope you get the idea).
If they respond with "wholesale what?" they get a cookie and are sent away.
if they respond with "Now that would be telling"... bingo... invitation to speak with line 2 is given.
Now we have a way of judging response from the question in 5 seconds that will tell us if hey are serious or not (worth spending time on).
Moral of the story?
You need to design your marketing to ensure the right responses are being taken ... and then make make sure you track em.
So in a way you have to design your marketing to "filter out" what you don't want, rather than "filter in" what you do.
There is no real good way. You basically have to make them an offer.
Now, your offer has to be irresistible in the sense that it gives your customer no excuse except to buy your product. This is tough to do.
You can look at trends in what gets traffic, what questions people ask and the comments they leave but there is no real way to know that a buyer is serious until she lays her money on the table.
That's why you need to make them an irresistible offer.
Over time you will find indicators, but it is specific to your market and what you are selling (eg. if you are selling cars, doing a test drive or haggling on price might be your indicators). I think the best thing to do would be confirm their ability to buy, rather than their intention. You can make sure they have authority to spend the money, and hopefully even ensure they have budget available to do so.