As a team leader, How do you identify employees’ strengths and weaknesses to assign duties?

By assigning them the same tasks and evaluating each employee's competence in accomplishing the task. You need to test each skill alone so as not to mingle with you it.

I support too Mr. Ali Hassan's answer

I support Mr. Ali Hassan Mohammed Saleh answer. 

Thanks

I support my colleague Ali's answer

I agree with Mr. Hassan answer.