Magento 2: How to Check if a Module's Installed

Maybe you are looking for this?

lib/internal/Magento/Framework/Module/Manager.php

/**
 * Whether a module is enabled in the configuration or not
 *
 * @param string $moduleName Fully-qualified module name
 * @return boolean
 */
public function isEnabled($moduleName)
{
    return $this->_moduleList->has($moduleName);
}

Also there is method isOutputEnabled in the same class. depricated since v. 2.2.0

Example of usage:

if (!$this->_moduleManager->isEnabled('Magento_Reports')) {

Part of goal of using composer is you can ensure dependencies exist, or else module won't install. However there is this "disable" output mode that is a little weird, but is more like "quick, it's broken, turn it off until we can fix it properly".

So the concept of is module enabled is not quite the same in m2.

Having said that, I am pretty sure there is such an API, but it might not yet be marked as public/supported with @ API annotation