Currently AGOL's Hosted Feature Service (and Hosted View FS) Overview pages list all related layers and tables but do not show that there are DB relationships. To my limited knowledge, you either have to know they are there or discover them through Attribute Expression widgets (popup - editors), ArcGIS REST Administrator Directory (JSON in "Update Definition"), etc.
It would be very helpful if the Overview page would:
Display which layer/table is related to which in perhaps a tree structure (great for nested relationships).
Show Relationship Class attributes such as:
Cardinality (1:1, 1:M, etc.) - a must
Primary and Foreign Keys for each relationship - a must