Get list of all the methods you can run on specific entity

To get list of methods you can run on certain types of entities you can get the list by loading this entity and then running get_class_methods on it so for example

$coupon = \Drupal::entityTypeManager()->getStorage('commerce_promotion_coupon')->load(3);
ksm(get_class_methods($coupon));

will list all the methods you can run on commerce_promotion_coupon entity.

Some other ways to get list of methods would be by namespace, so you can get methods for any classes, even vendor projects

ksm(get_class_methods('PhpOffice\PhpSpreadsheet\Reader\Xls'));

or the nices way to get output would be 

$class = new ReflectionClass('PhpOffice\PhpSpreadsheet\Collection\Cells');
$methods = $class->getMethods(ReflectionMethod::IS_PUBLIC);
ksm($methods);