How to diff between 2 users with uid 0?

Hello,
I created a new user "rootNew"
After creation I manually change the file /etc/passwd and gave the new user "rootNew" uid 0.
Now I have 2 users with uid 0 (root,rootNew) how can I know which user is log in the system?
"whoami" command return "root" for both users.
Thanks,
Uri