Even if your brand is protected by a trademark, it's still your responsibility to track the market for instances of infringement. In some cases, loyal customers will report infringement; however, depending on you or them stumbling upon copycats is not advisable.
Some IP offices will notify the owner if the examiner evaluates a new trademark application and believes it to be confusingly similar to the owner's trademark, lower the chances of infringement posed by new trademark applications.
For a more comprehensive approach, trademark infringement can be monitored by so-called trademark watch services. These scour the web, online marketplaces and trademark databases to discover potential violations.