How to obtain an evidence of teacher effectiveness when I never was evaluated?
If someone is asking you to teach classes before you've had any experience doing it, then I'm going to guess that they don't expect you to produce a full dossier.  In that case, letters of recommendation from the people that invited you to give guest lectures, or people who saw them, might be sufficient.  You might get some clarification on what's expected by asking the person who's asking you to teach the classes.