Ever wish you could tap into unseen sales opportunities? One of your biggest sources of info is the customers you already have, but sending them customer surveys will only tell you so much.

Instead, make a follow-up phone call and ask six simple questions that will tell you where to find the next big opportunities. That's the advice from revenue coach Kristin Zhivago who has used follow-up interviews with thousands of customers to increase revenue for her clients.

It's important, she cautions, to ask the questions after the sale is concluded, not during the sales process. "While you're selling to someone, they will not tell you what they're really thinking," she says. "They're in skepticism mode, and not on your side, so asking salespeople to gather information about what customers want is a dead end. This is why I call after they've made the purchase when they have a vested interest in your success."

Zhivago asks many questions to tease out detailed information from customers, but these six will give you a good start.

1. What has your experience been with our company so far?

This gives you a chance to find out what your strengths are, as well as any areas for improvement. "They may say, 'You have a great product but the service has been terrible,'" Zhivago says.

2. What is your biggest problem related to this product or industry?

"Whatever the answer is, that's your biggest opportunity," Zhivago notes.

3. What trends do you see affecting this industry in the future?

Customers may be able to spot significant trends before you can, so this question may also point the way to some new opportunities.

4. If you were searching for this product and didn't know about it, what words would you type into a search engine?

This question will help you check whether you're using SEO and search engine marketing effectively. "The answers tend to be surprisingly consistent among customers," Zhivago reports. She also recommends a follow-up: If you searched on those words and got too many results, what filtering criteria would you use?

5. If you were made CEO of this company tomorrow, what's the first thing you would focus on?

Learning what your customers think your top priorities should be will tell you a lot about the threats and opportunities for your company--and it's likely that their idea of your top priorities won't match yours.

6. Is there anything I haven't asked you but should have?

Whenever you interview anyone for any reason, this should always be your final question. Most of the time, the answer will be no. But sometimes you'll hear ideas, complaints, or suggestions that will point to opportunities you never would have that of by yourself.