There are various ways to check out information about external dependencies in a model only if the related model is also checked in in the repository.
Select the Check Out Dependencies check box in the Check Out dialog box when you check out a document.
Select the Check Out Dependencies check box in the Repository tab in the General Options dialog box to systematically check out external dependencies.
Select the Check Out Dependencies check box in the Check In dialog box if you choose to check out the model after check in.