Since Gelatinous Cubes have the transparent trait, do you need to do an action perception check to spot it or you could use your passive perception?
Transparent trait: Even when the cube is in plain sight, it takes a successful DC 15 Wisdom (Perception) check to spot a cube that has neither moved nor attacked. A creature that tries to enter the cube's space while unaware of the cube is surprised by the cube.
Whether or not to ask your players to do an active check or just use their passive is up to the DM to determine. The only direct answer is if the player says they are looking for something, in which case, you can have them do an active check for something they are actively doing.
Otherwise, using a passive score or asking for a player to do an active check is purely up to the DM and there is no direct rule regarding this.
Either way, monsters are also here to tell a story and not just be a bag of XP. It's okay for players to stumble into it, that's why it's in the stat block! But if you want to as DM, you can also try and provide some clues that something is amiss. If they ask you to do a check, then you can have them do a check.
What the Transparent trait means:
Normally, a creature cannot hide in plain sight. The transparent trait allows the creature to be hidden "even when" in plain sight.
What Passive Perception means:
DMs can use passive perception when they want to secretly determine whether the characters succeed at noticing a hidden monster.
If a DM wants to secretly determine whether a character notices a hidden gelatinous cube, then apply passive perception. If the DM is not secretly determining whether a character notices a hidden gelatinous cube, then dice are being rolled to see if the character notices the hidden gelatinous cube, because there is still a chance the character notices the cube even if he is not searching.
Crux of the Question:
Of course, a player character can choose to use an action to actively search for signs of the presence of a gelatinous cube. But the question here is whether a player character needs to spend an action to notice the cube.
Answer to the Question:
The answer is: no, not necessarily, meaning that a player character gets an initial chance to notice the cube without having to spend an action. A player character could use passive perception to notice a gelatinous cube. By RAW, the DM uses passive perception to replace a roll, meaning that if the DM is not using passive perception, then the DM is using a roll (that does not require an action) to give the player character a chance to notice the cube without searching for it.
Example: Player character Apdog has +5 perception. Player character Dojomon has +2 perception. Player chracter Merkle has a -1 perception.
Merkle is following Apdog as he walks down an open corrodor containing a gelatinous cube. The DM wants to secretly determine if Apdog notices the gelatinous cube. Apdog's passive perception is 15 which is enough to pass the DC 15 check to notice the gelatinous cube. Apdog notices the cube and opts not to walk into it. Merkle's passive perception is 9, so Merkle fails to notice the cube... however, since Merkle is following Apdog, Merkle stops and also does not walk into the cube. The cube does not move or attack, and Merkle still can't see the cube. Apdog tells Merkle something is there and Merkle decides to actively search for it. Merkle uses an action and rolls 13-1 = 12. Still not enough. Merkle can't see the cube!
But wait... Dojomon is walking down the same open corrodor but from the opposite direction, traveling to meet up with Apdog and Merkle. Dojomon is not searching. The DM decides to roll openly to see if Dojomon notices the cube instead of secretly determining if Dojomon notices the cube using passive perception (because reasons, suffice it to say the DM wants this roll to be made openly for all the players to see). The DM calls for an open roll and if Dojomon rolls high enough he will notice the cube and not walk into it, but if Dojomon rolls low enough he will fail to notice the cube and walk into it. Dojomon rolls a 12 +2 = 14, which is not enough to beat the DC of 15. Dojomon walks into the cube!
A Gelatinous Cube, unless newly 'spawned', would have items floating inside it that the Cube could not, and cannot digest. Clothing, flesh, leather armors, strapping, belts, shoes (unless horseshoes), in fact anything 'organic' will be consumed, but most metals and magic items will not be 'consumed' by the Cube. Thus, a gelatinous cube filling a 10-foot wide corridor will, in the VAST majority of cases, have items floating within its body. So, seeing items seemingly floating in the air will 'give away' that the cube is there. Also, things recently enveloped by the cube, will be only partially consumed, and again, be a 'tip off' of the cube's nature.
