Cover it at the briefing. Confirm that the applicant understands the icing limitations appropriate to the aeroplane being flown for the test. Ask him/her how he/she would avoid icing; check understanding of how to escape from it. Then state that the test will be conducted using 'foggles' (not those bŁoody stupid and higly dangerous bits of tin that the CAA insist upon for IRs), that you will not direct the applicant into possible icing conditions and that you will advise him/her if unexpected icing occurs.