The evidence that the switchman uses is their travelers' look. He says that they are sleep and that the ones that are not sleeping are yawning.
Explanation:
The reason behind this fragment from the little prince is that the little prince asks him what is his function.- Then he answers that he provides direction to the travelers. Then the little prince asks where they head at. But the switchman says nowhere. The little prince asks then what are they looking for and our argument is brought. The switchman says that they don't look for anything. They are sleep "motionless" they feel nothing and then both analyze that only the children know what they want because they cry when someone takes out their toys from them. meaning that the rest of the travelers are not attached to anything or have feelings for anything.
