I'd have the person trying to peek roll Stealth. I'd describe what they see based on their passive perception, and only demand a Perception roll if the monster is actively hiding (possibly because it heard the guy approaching from a botched Stealth and is preparing an ambush of its own).
I advise this because the point of meaningful failure in this case lies in the Stealth check: Whether or not the monster sees you at the same time as you see it. Especially as you know the characters are doing this to set up an ambush.
The players don't need to be told whether the Stealth check succeeded or not. Instead, you narrate what they see when they peek, which might depend on that check's result: Maybe the monster is currently looking up in surprise at the head poking around the corner, or maybe it's still busy making a rope out of human hair. Or maybe there isn't a monster at all.
If the monster is making enough noise that people wouldn't need to round the corner to become aware of its presence, I'd also handle that with Passive Perception, as something done even before the Stealth check is rolled.