Q:

How can I check if a module is enabled?

In Drupal 7, you can use module_exists() to check whether a module is enabled (as well as whether it exists).
For example,
if (module_exists('devel')) {

How can you check whether a module is enabled in Drupal 8?

A:

Just like Clive mention....

service would be preferable to using \Drupal (assuming it's an option in current context)

Here is an example of checking if a module is enable but using a service. 
$moduleHandler = \Drupal::service('module_handler');
  if ($moduleHandler->moduleExists('devel')){
     // Code Here
  }

A:

Like this: 
  if (\Drupal::moduleHandler()->moduleExists('devel')) {

