I like rolling against Wisdom, and you need to roll low.

One advantage is that if they are trying to spot something real, and they roll low, they successfully see it. But if they are saving against an illusion, and they roll low, then they see the illusion as real. So the process of rolling a saving throw doesn't inherently tell the players that there is an illusion. This protects the game from (one aspect of) meta-knowledge.