You use a sales technique called "qualifying", and when you figure it out, you do it all the time, and not just with potential customers, but also with potential investors, candidates, and even reporters.
(This is part of the art of sales that many founders today refuse to learn.)
To qualify a prospect (potential customer), at the end of every interaction, you should determine from them what has to happen from here for them to pull the trigger. You do this by asking questions, and testing the answers -- have them lay out their process for making the decision, have them identify the steps that have been reached and the steps that have not, have them identify other people in their organization who can block the purchase, and so on. And you do this over and over again until you get the sale.
If the prospect can't lay out the logical sequence of steps from here to close, then you need to qualify them out and stop spending time with them, because they're not going to buy.
Two interesting corollaries:
* Yes, there are a LOT of people in a LOT of companies (as well as a lot of potential investors, potential recruits, etc.) who will spend a lot of time with you even though they can't ultimately pull the trigger. A lot of people like to windowshop, and a lot of people have nothing better to do.
* Like a lot of sales techniques, this sounds to engineers like it might be hard (will the potential customer tolerate all those questions?) But any serious potential customer will have a very easy time answering the questions, and will respect their potential vendor for being professional in the sales process.