Short answer
Run a whois lookup to see the registrar and, sometimes, the registrant. Most owner names, emails and phone numbers are now hidden by privacy law or a privacy service, so the record often shows only the registrar. When it does, contact the owner through the website itself, the registrar's contact relay or the abuse address.
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What a whois lookup actually shows
A whois record is the public registration record for a domain. It names the registrar the domain was bought through, the dates it was registered, updated and will expire, its status codes and its name servers. It may also list a registrant, the person or company that registered the domain.
The registrar and the owner are different things. The registrar is the company the domain was bought from. The owner is whoever holds the registration, and that is the part you usually cannot see.
Why the owner is usually hidden
Since European privacy law came in, registrars and registries removed personal details from the public record for most domains. You will often see the registrant name, email and phone replaced with a note that the data is redacted for privacy.
Registrars also sell privacy or proxy services that put their own details in place of the owner's. This is normal and is not a sign that a site is suspicious. Many legitimate businesses use it.
Organisation details sometimes stay public. Some country registries still publish a registrant organisation, so it is worth looking before you assume it is hidden.
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How to look it up
Type the domain into a whois lookup and read the registrant section. Our free lookup shows owner and contact details when the registry publishes them, and says plainly when they are hidden instead of guessing.
Look up the bare domain, such as example.com, not a full page address.
How to reach the owner when the details are hidden
Start with the website. A contact page, an about page, a footer, a privacy policy or terms page usually names the business. In India, a legal page often carries the company name and registered address.
Many registrars forward messages to the owner without revealing who they are. Use the contact form or relay email the record shows, and keep the message short and specific.
For spam, phishing or abuse, use the abuse contact in the record. It goes to the registrar's abuse team, who can act on it.
For a legal or security reason, ICANN runs a request service for non public registration data. It is for legitimate requests, and the registrar decides whether to release anything.
If you want to buy the domain
Use the registrar contact or a domain broker rather than guessing at the owner. A broker can approach the owner without you revealing who is asking, which often keeps the price lower.
Check how long the domain has been registered and when it expires first. A domain close to expiry may become available without any negotiation.
If someone is using your name or brand
Trademark owners have a formal route for domains that copy their mark. For global domains it is called UDRP, and country domains have their own, such as the dispute policy for .in. Gather the whois record, the dates and screenshots first, because the registration date is evidence.