In any game I ran, they'd have to be pretty intelligent. It's not like Bless or Bane creates an obvious in-game effect. I might see an opposing cleric maybe noticing or someone with super-tactical "Admiral Thrawn" type insight. It's like being made more or less lucky, not supercharged with glowing power.

Otherwise, I just feel like I'm using GM metagame knowledge that most GMs don't like players using.