php bin/magento deploy:mode:show

protected $_appState;

public function __construct( MagentoFrameworkAppState $appState )
{
    $this->_appState = $appState;
}

public function doSomething() {
    switch ( $this->_appState->getMode() ) {
        case MagentoFrameworkAppState::MODE_DEFAULT:
            // Action for default mode
            break;
        case MagentoFrameworkAppState::MODE_PRODUCTION:
            // Action for production mode
            break;
        case MagentoFrameworkAppState::MODE_DEVELOPER:
            // Action for developer mode
            break;
    }
}