I would ask both guards which road the other guard would advise me to take. The one that tells the truth would tell me the wrong road, for the guard of the wrong road would, as a liar, advise me to take his. The one that lies would also tell me the wrong road, for the guard that tells the truth would advise me to take his road and the liar guard must lie. Once I have identified the road that leads on forever, I simply take the other.