In the books, they all could tell something was up with Frodo and refused to let him go alone. In fact, Sam was actually dropping plenty of eaves, dispite his claims to the contrary in the movies, and they puzzled it all out before Frodo even left Bag End. (Fatty Bolger, another co-conspirator who did not make a movie appearance, decided to stay behind to keep up appearances so that the mission stayed a secret as long as possible. He bought them precious time when the Nazgûl were in pursuit, and later played a role in the resistance against Saruman prior to the Scouring of the Shire.)