As the title says, I'm not sure exactly how to approach this encounter. I don't need help on where to place it and what sorts of traps to use it on etc., just the initial do you spot it.
In the olden days it was something like "90% undetectable," it was a one-off % resolution rule. The classic game was full of stuff like that. But the point wasn't the roll, the point was to describe to the players:
"You see a skeleton floating towards you down the corridor!"
And see what they did. If one of them rushed forward to attack it :schplooorgh!: paralyzed. Gotchya.
Once you've done that you never need to use a Gelatinous Cube again.
- Do you ask for perception checks and alert the PC’s that there is something to spot?
- Do you rely on Passive Perceptions and anyone high enough just spots it?
- Or do you rule that if no-one is actively searching then they can just walk right into it if they move into its space?
You're the DM. In 5e you decide how to resolve any given action. If the action is walking down the corridor, when you know there's a Gelatinous Cube filling that corridor, you decide whether to call for a check, make a check vs a passive score, or narrate the result...
... as a general rule of thumb, I personally think it's a good idea to either narrate a result or call for or make a check vs a DC. I don't think comparing two passive scores is worthwhile (so avoid just comparing passive perception to a DC to detect), and, on the other extreme, feel the same way about the very swingy contested check. If you don't want to call for a check, make a check using passive perception as the DC or just narrate what happens.