you want to check it in a calculation field which is in the same item as the relationship field? Let's say the relationship field in App A links to App B which has a title field named "Title". In each item the title field always has a value. Then you can do in the calculation field in App A:
@All of Title.length
This shows the number 0 if there is no B-item related, or 2 if there are 2 related..