How to know a supervisor's working style before joining his group? I have recently joined a research group, where everything was excellent until two weeks of my joining.

My supervisor was very friendly when I met him for the first time at a conference. He was very interested in conducting work similar to mine in his lab. He showed interest in the subject and insisted that I make an application for a vacant position in his group. 

He was very happy when I cleared all formalities (completed the application, was shortlisted, performed well in the interview and received an offer letter). After joining, I felt that I had joined a very good group that could help me enhance my research skills, as my colleagues are very friendly and helpful.

However, two weeks later, when I met my supervisor to discuss my work, he suddenly gave me many difficult tasks, many of which are very new to me and out of my skillset and told me that everything should be finished within another two weeks. He also told me that I should do everything by myself and never disturb my colleagues. 

When I discussed this with my colleagues, they told me that he is very strict, not a good person, and he intends to dominate.

My question is how would I know about these things before joining his group? 

For now, I can’t imagine how to continue to work in this group.
...never disturb to other colleagues. When I discussed with other colleagues...


Good for you!


  My question is how would I know about these things before joining his group? 


By talking with other members of the group, away from faculty, during your interview visit (or some other time before accepting the offer).

Conversations with students outside the professor's group would also have been useful. Also with the clerical staff. Even if unhappy group members themselves are hesitant to talk with strangers, for fear of reprisal, people know who the unhappy research groups are.

If the professor didn't actively encourage such conversations, that's already a red flag.