Jana Sattainathan explains how to get a user’s effective permissions:
Microsoft has provided a function named fn_my_permissions that gets the permissions for you. It has multiple classes of objects for which you can get permissions. We already know about SERVER, DATABASE and OBJECT but there are a lot more. To get that list of classes, let us use fn_builtin_permissions function
The one thing I wish this function did was let a sysadmin see another user’s effective permissions without using EXECUTE AS; I’d like to be able to apply that function to sys.logins and get an exploded list for reporting.