Actually, there are some plumbers that I'd trust more with my car than some mechanics.
If I want to know authoritatively who is and isn't licensed in a profession, I'll go to the authority: maybe the authority has issued a certificate to the person that they hang in their workplace. Maybe they'll have a little wallet card. Maybe they'll be listed in an online directory of practitioners.
In any case, I shouldn't have to take the person's word for it that they've been deemed qualified to do what they do.
If we're going to extend this analogy to Christians, then I shouldn't accept anyone's word that they're actually a Christian unless God has expressed to me in some sort of clear and obvious way that the person is a Christian. Maybe a little wallet card reading "I certify ______ as a True Christian(TM). Signed, Jehovah. This card remains the property of the issuer and must be returned on request."
Does that sound reasonable?