think
yoda always uses the force, he can feel great pain in anakin when his mother dies and he kills the tuskins. you have to accept that fact that yoda went into hiding and could feel through the diminishing force that less and less jedi were alive, and soon he could only feel obi wans presence. i believe he even mentions to luke he has been watchin him for a long time? i might be mistaken but some kind of reference is made to him sensing luke.
and even if the force doesn't make u believe then realize obi-wan has conversation with yoda after he has become one with the force, so before yoda died old ben who was living in an area where he could recieve news of all survivors dying would tell yoda eventually.
do not try to look to deep into it