How do you do this? It may sound simple but ..... ask them.
But don't leave it at that. You need to measure their response to what you ask.
In my opinion, there's nothing better than the Net Promoter Score question to accomplish this.
It's so simple to ask in surveys, or better yet, within your website or application. It's just asking customers to rate you on a scale of 0-10 and give them a chance to offer feedback.
"How likely would you be to refer a friend or colleague to
The important part (obviously) is acting on the feedback that you receive and letting customers know when you've acted on the feedback. It's a fantastic tool.
You can find this tool here: NetPromoter
That should be enough to get you started.
Remember .... ask, measure, adjust, deliver. Do this and your customer satisfaction will go up and your business will grow.