Thanks for A2A.
It is not easy; you need to interact in such a way that the knowledge available with the other person appears as tip of the iceberg. Then you guess whether it is iceberg underneath or it is merely a floating piece of ice. Usually, you may find both type of people. Then you have to analyze carefully. You need to spend enough time, more the time together better would be the findings.
Questioning is a good way to extract knowledge and, in the process, know their level. Sometimes, one finds the type of a person like he/she is a go-getter, research-minded, generalist, master of such and such trade, good writer, better orator etc.
If you want to know what I know (a personal question), frankly, I am still confused about that. I do not know what I actually know! But I do not think asking me personally that question is going to be of any consequences, so forget it; I have added it perhaps for covering up this aspect.
Thanks for reading.