How to check who the domain is registered to?

Method 1 - WHOIS request

For this, simply use a profiled WHOIS domain checker tool.

Such a service will give you a lot of useful information about the domain. Where, among other things, can be and contact information that the owner specified during registration. 

However, most registrars and providers allow you to hide personal data, then in the fields of personal information will be contact information technical registrar. 

Method 2 - Contact the registrar 

Even if the owner of the domain has hidden the data, you can always write to the registrar itself. The company that registered the domain you need exactly has the contact details of the owner. If your arguments for contact will be convincing, the registrar can provide your contact information to the owner and he will contact you. 

This may work if you want to buy the domain or want to report a vulnerability. 

Method 3 - Write to the owner himself

Even if personal information is hidden, there are a number of technical mailboxes: 

admin@site.com
administrator@site.com
info@site.com
contact@site.com
dev@site.com
test@site.com

Such boxes are created for debugging the work of mail or are needed for verification when issuing SSL certificates. Write to all of them, perhaps one of the addresses will find an answer. This way you will know who registered the domain.