The most reliable starting point for validating domain ownership is to use a port 43 whois service, a requirement in the ICANN registrar agreement to be provided to the public by every accredited registrar. Most large registrars offer a whois service lookup straight from the homepage, while others have it linked under their domain tools or as a link in the footer of each page. Best practice is to look up the domain at your choice of whois service, and then from the results lookup the domain again at the whois service of the registrar listed for that domain. While all registrars data should be up to date, it is best to check at the registrar in control of the domain.
It is a requirement for whois contact information to be kept correct and up to date by the domain owner. If an email bounces, a postal mail comes back as undeliverable, or calling the person at multiple times renders no answer, be very cautious.
If you have any doubts whatsoever that the domain in question is owned legitimately by the person claiming ownership, there are several good avenues to further validate ownership. Use some common sense as well. Domain scammers are notorious for high pressure selling and prices too good to be true. They'll try to offload the domain before the legitimate owner even realizes it's gone.
Use the ownership information in the whois contact section. Email the address listed. Ask them to confirm their online identify or some detail you've spoken with them about previously. Call the phone number listed. Send a certified letter to the address.
Is the technical contact information different from the ownership information? Scammers make mistakes too, maybe they were lazy and didn't bother to change the technical contacts. Utilize that information.
Hit the search engines, using phrases like "example.com stolen", "example.com legal", "example.com sold", "example.com for sale". Domains that have been stolen recently are often posted to the legal sections of domain sales forums, or blogged about by industry watchers.
Look at the date the whois information was last updated and modified. Has the domain been sold recently? Has it had the same owner for the last 10 years?
Find out if the seller has a user account on any large domaining forums. Ask them to send you a private message from their user account to verify they are who they say. Many such forums offer reputation systems with feedback from previous sales that user has completed. You can also simply post and ask if any long-time members there would vouch for the user.
None of these indicators guarantee the seller is a scammer, just as none of the methods listed above validate domain ownership with absolute certainty. These are simply the most common techniques scammers employ. History has proven that if you use some common sense, thoroughly research both what you're buying and the seller, and watching out for these warning signs you'll make a very difficult target for a scammer!