Compare value and set custom object

Hello,

I try to automatically set a “under support contract” from serial number field filed by customer.

To do this :

I have a custom field “Serial Number” in the customer ticket form

I want to compare the serial number with the serial number in the custom object database. This on is linked to a contract (custom object too) with time expiration. if the contract is still running, set the custom value “under contract” to True

Do you have any solution to do this ?

Thanks in advance,

BR David