Spending time is one option, but naturally however bad he/she may be in actual life, he/she will try to be modest in these meetings.
So try to know his background, his habbits, nature from some sources that can be trusted, common links between you and him that you trust. Do this without his/her knowledge.
That is neccessary belive me.
Says Krishna Indian
05 Nov 01:39 pm
This answer is chosen as the best answer for this question.