I have my own suspicions that there are blocked users of my website that use Teamviewer to login to my site from their friends' pc. Is there any way to identify if the one who visit a website is using the Teamviewer at the same time?
No you can't.
Teamviewer is a Program which "control" the other PC. To find out if Teamviwer is running on a PC you must have access to the Computer, physically or with a remote control program like Teamviewer or Remote Desktop.