1. Go to the scene, after fleeing the Cloverfield parasites in the subway, where the survivors find the room and close the door as the parasites attempt to enter (sometime around 43:47 minutes - varies depending on movie viewing program).

2. Pause the move just at the beginning of the flashback after this, where Rob and Beth are on the train at Coney Island.

3. Go frame-by-frame (or just keep pausing and hope you get lucky) and during one of the pink flashes shortly after Beth smiles at the camera you should see a still of the dinosaur from the monster movie "The Beast from 20,000 Fathoms".