How do you recognize an external user?

[Pieter Veenstra]

I created some code to check if a user is internal or not by checking for #ext#@ in the username:

public bool IsInternalUser
        {
            get
            {
                return this.Username.IndexOf("#ext#@") == -1;
            }
        }

Is there any CSOM code that does something similar?

CC: @Vesa Juvonen