It is a great thing that there’s DNA test. I am trying to imagine what people would do if it weren’t there; how they would know that they have an unfaithful partner.
Prior to DNA testing, blood typing was the most reliable paternity test. Blood types are hereditary, so a child and parent having incompatible blood types can prove they're not really related. Unfortunately (well, fortunately for people needing blood transfusions), most blood types are common enough that having compatible blood types doesn't prove two people
are related, so there was always uncertainty in such cases.