I have known buyers to interview high end contractors which they carefully turn in to a consultation. They might never be there to pay and are simply interviewing you for consultation on the topic and to get multiple options. This is unfair to you because you are spending time on a client that does not intend to contract.

You can recognize this when you see the client is starting to ask questions on how you would perform the job. While you can give 1-2 very generic tips or in other ways prove that you are the guy who can get it done, when it comes to this, you should ask for consultation fees. Once you ask for consultation fees from that point onward, it is completely clear - a willing client will hire you for a consultation get get all options they can benefit from or simply contract you for the actual task. A window shopper will back out.

The only thing to be careful about is, not to ask for consultation fees too soon into the conversation as a legitimate client may be curious about how good you are at what you do. This is something you can prove in multiple ways.