Checking The Other Users got all Required Privileges are not by System Admin

Hello All,

I am System Admin  for organization ,Organization have  So many user with different roles,But I changed One User Privileges added Some Security Role him , Now I  want Know,he can able access the  Changed privileges are not, But problem is I don't have his/her Credentials,Without physically login using there  "User Name and Password", How i can verify Privileges are work for him .