Using below example we can check if module is installed or not :
<?php
if (\Drupal::service('module_handler')->moduleExists('comment')) {
echo 'Module Installed';
} else {
echo 'Module not installed';
}
?>
if (\Drupal::service('module_handler')->moduleExists('comment')) {
echo 'Module Installed';
} else {
echo 'Module not installed';
}
?>