Your fan base tells you so
Obviously, the easiest way to know that your brand is becoming uncool is for someone else to tell you so. This is usually painful to hear. But hearing it is much better than having people talk behind your back. And luckily for brands, there is no shortage of consumers out there today who will gladly tell you when your marketing or products or customer service has become, well, totally lame.
The Gap is a well-trod but classic example of this. In late 2011, The Gap tried to change its logo, presumably to demonstrate that it was keeping pace with the times. But its customers were not pleased -- and they let the brand know via a torrent of social media criticism. As it turns out, The Gap's customers liked the classic logo. Classic is cool. Progress for progress's sake is not.
How to make it cool again
Listen and respond. It's as simple as that. Make sure you have your ear tuned to the social media sphere. Ideally, this would entail a robust social listening platform, but in the least, set up Google Alerts and have someone keep an eye on Twitter and Facebook for you.
In The Gap's case, the brand did it right. It returned to its old logo and hasn't looked back.