Script to find if entry already exists

[finalcloud]

I am working on a table called Personal_Device that has an Owner and a Serial field. What I want to happen is that when a value is entered into the Serial field, you either click out of the field or a button to make a script run. I would rather do it without the button, but if I have to, I will.

I want the script to take the value in the Serial field and compare it with a Key field in another table called Device_Info. If the value does not exist, I want the script to create a new record in the Device_Info table that autofills with the value of Serial in the Personal_Device field.

Any help would be much appreciated.