Since most retailers focus more on selling products and stocking shelves than on delighting their customers, they more often than not miss the fact that they have an unhappy customer. 
Most unhappy customers don't bother to complain; they just leave the store and never come back. Meanwhile, store management and staff chugs along believing they are giving good customer experience when the opposite is true.
A study by Forrester Research's Marketing found that while 80% of companies reported providing a superior customer experience, only 8% of consumers who were asked the same question said they had received a superior experience. This is another example of the experience gap.
Another study, this one conducted by M/A/R/C Research and National In-Store, found over 16% of consumers said they would stop visiting a store all together as a result of a bad customer experience. Over 95% of consumers indicate sales associates are very/somewhat important, with two-thirds indicating they are very important.
When retailers focus on creating memorable experiences they're less likely to create unhappy customers but on those occasions they do fall short they're more likely to know it, even when the customer doesn't tell them outright.
These are just a few of the ways your staff can tell a customer is unhappy without the customer actually saying it.
1. They state their complaint in the form of a question. I was out to dinner with my children last week and it took an hour for us to be served in a restaurant that should have served us in 20 minutes at the most. When the waitress arrived with our food I asked, "So does it normally take an hour to be served here?" Without missing a beat she said, "Nope. We just had a few big parties place their order before yours" and off she went. It's clear she didn't hear my question as a comment or complaint. My ten-year old did, though. Her comment: "Boy, dad. She's clueless." Yes, Kate, she was. And unfortunately she's not alone.
2. They abruptly stop shopping and leave the store. This can happen for numerous reasons, but it's often the result of an unhappy or frustrated customer. I remember once a customer stormed out of the store and I said to the store manager, "Wonder what's wrong with her." He looked at me and said, "Well, I don't know, go find out." It had never occurred to me to go after a customer and ask if there was a problem. Sure enough, when I found her outside the store she was upset with one of the associates. I invited her back into the store and saved a $500 sale.
3. You overhear one customer telling another customer that they're not happy. If the customer said it loudly then you can easily walk up to him/her and offer to solve the problem. If it wasn't said in a "carrying" voice you may have to take a different tack since you don't want to appear to be eavesdropping. Either way, this one's a gift. . . and it's not polite to refuse a gift.
4. The "I changed my mind" product return. A lot of customers do simply change their mind about a purchase but that still doesn't mean they aren't disappointed or unhappy about it. There may be days you find this hard to believe, but most customers won't complain about a return. By politely asking one or two follow-up questions to the "changed my mind" comment you're likely to uncover either the real reason for the return, or at the very least how the customer really feels about the product.
Dealing with an unhappy customer is never the most pleasant part of our job but it sure beats losing a customer and their future purchases.