I found that the engine provides EPrivilegeResults for AccountTypeFailure (Premium account required). However I didnt find a way to extract the current privilege from the corresponding EOS platform subsystems.
There is the FOnlineIdentityGDKPtr which can be used to GetUserPrivilege, however the UserPrivilege is not the EPrivilegeResult so thats not quite it.
How would one check for either a XBox Live Gold or PSN Plus subscription to gate online play (or give an appropriate error message to the player)?