The Gemara says that when Rabban Gamliel was removed from his post - they also removed the shomer from the door. The guard's job was to keep out anybody who was not "tocho k'baro". How can the shomer determine whether or not an individual was really a good guy? It is not like he could ask to see his driver's license? What was the process used to exclude talmidim that were not what they appeared to be?

I heard once a P'shat that he turned all away, but only those who were sincere found a way to get in. The others had their excuse why they can't learn and left.

I think that Tocho Kibaro, isn't that you couldn't tell at all from the outside (as it;s all in the mind) but rather could be told by his actions, more than his outward apperiance. In Yoma 72b it brings that a Talmud CHachum who's not Tocho K'baro is not a Talmud Chuchum andis disgusting. From there it talks about those who learn and doesn't have Yiras Shamayim. So I think that might be the idea. That he's not K'baro i.e. of when he's learning, but acts differently in other situations.

Thank you. I like that idea. It is also good musar! We need to make sure our learning is not simply an intellectual experience!