What if a wife or fiancée asks her husband or fiancé: "Have you committed adultery or not?" and he has committed adultery and concealed himself with God's concealment, should he lie or is that the appropriate response?

A Muslim is not permitted to disclose sins committed privately between himself and his Lord. Whoever does so has committed yet another sin and has torn the veil of concealment that Allah Almighty had placed over him. If his wife or anyone else asks him about it, he should not inform her. He is permitted to use tawriyah (ambiguity) or even lie if there is a need. As for making frank disclosure a condition in the marriage contract, this is neither a requirement of the contract nor does it benefit the woman in this particular matter, so long as the husband has repented sincerely. This condition is not obligatory to fulfill and may lead to speaking about a forbidden act of disobedience.