According to other forums no-one is 100% sure but doing all these, you will get it at some point:
*When you talk to Max, pick the Random option multiple times and he'll mention several different characters from previous seasons like The Devil, Santa Claus, and The C.O.P.S.
*Talk to Skun-ka'pe at the beginning of the game multiple times. Eventually, Sam will offer him a bottle of Banang (Season 3 and Tales of Monkey Island). [Thanks to dab198]
**IMPORTANT** Must be done BEFORE using the radio in the diner.
*One of the dialogue options with Momma Bosco (the first one asking about the Clone Master) will elicit a reaction from Max about the Soda Poppers (Season 1).
*Examine the door on the second floor of the room twice. The second time Sam will mention "not being able to hit the road" while the clones are running around. (Sam and Max Hit the Road) *Needs to be confirmed*
Cloning Pod area
*Examine the Dispenser (from Team Fortress 2) in the corner of the room. If you do it enough times, Sam will hit the dispenser with a wrench until it dispenses some Banang. When you try to get it, the dispenser will retract. Afterwords the item is unusable.
*Examine the Tentacles after you've spoken with Norrington and heard about the Cthonic destroyer. When you examine them, Max will mention wishing it was a friendly green tentacle (Day of the Tentacle).
*Use Sam's Gun on the clone pods. The things that the main computer says after a pod is shot reference a number of things (THX, 2001 A Space Odyssey, Les Miserables, The Hitchhiker's Guide to the Galaxy, "In the Year 2525" by Zager & Evans, "58008" spelling Boobs when flipped upside down on a calculator, and Fahrenheit 451).
*When you use the Cthonic Destroyer on the Tentacles, Max will shout out "UNHOLY THIS!" (Tales of Monkey Island) [Thanks to Bucket]
*Examine the phone in Paperwaite's office twice and Sam will mention wanting to talk to his shrink, who is still on vacation. (Sybil from Seasons 1 and 2).
Statue of Liberty