That would be the same as saying how do you know your sifu is any good. If your sifu is qualified to teach iron palm he should also be qualified to administer the proper jow. Your immediate problem is finding a sifu you trust. Contact other Sifus in or associated with that lineage. Tit Jueng training and dit da jow go hand in hand. You can't have one without the other. If you don't trust your sifu's linament don't train in his method.