Well, there are many ways to tell this.
The first and most obvious was is to just do a quick WHOIS look up from any WHOIS website like whois.net and look at what shows up in the “creation date” area. That should tell you what date the domain was originally registered (unless the name was dropped and was reregistered again).
Other options are to try and get the creation date from a service like Domain Tools or to use free services like archive.org or hosterstats.com
Hope that helps.