Easy. I present you two universes, one with our cosmological constant and one with a slightly smaller but still nonzero one. The differences would only become apparent over distances greater than a few billion light years or so. Out planet, solar system or even the entire galaxy would be virtually indistinguishable. But inhabitants of both would wonder how the cosmological constant got cancelled out so precisely against the QFT vacuum over so many orders of magnitude when studying the sky. No bias required.