You can use Device Coverage Checker, enter the serial number of the SX20 and if it has an active service contract it will show you contract details. The description of the tool from Cisco's website is: "Check service contract coverage for your Cisco devices".
You probably don't have the service contract added to your Cisco.com account, see the message it says in the lower right.
Another method is to use the Cisco Licensing Portal, can search for the serial number, and you can see some contract details. Navigate to: Get Other Licenses > TelePresence License to Resend and enter the SX20 serial number.
If you know the service contract number, you could request access to the Cisco Service Contract Center, from there you can see the full details of the contract itself.
Since you're the reseller I'd assume you'd contact your Cisco Account Team and work with them to gett the contract associated with your Cisco.com account, so you can access CSCC to see the contract details, or they could tell you the details if you're allowed.