A hook that would check if the module is enable? I'm not looking for hook_enable (which would trigger when the module is been enable).

I have a module that would work alone (lets call it custom_module_1) and I have another module which is an add-on (lets call it custom_module_2) to that module to the custom_module_1. I want to be able to have a different behavior on the custom_module_1 if the custom_module_2 is enable.

So I'm looking for a hook that would be be using on custom_module_1 and would check if the custom_module_2 is enable.

Or how can I check if there is a module enable as a function to have a different behavior on another module ?
module_exists($module) will do that for you.  From the API:

Parameters
string $module: The name of the module (without the .module extension).
Return value
bool TRUE if the module is both installed and enabled, FALSE otherwise.