There are a couple places where you can find information on previous jobs you've done for a customer.
If you open a customer contact record you can filter the visible jobs. If you'd like to see completed, finalized, on-hold, etc.. This will show you a list of jobs that have already been performed and finalized for this particular person.
If you also want to see any jobs that are currently open for a customer, you can tab over to Open Jobs, this will show you a list of any jobs that are currently scheduled out or in progress for someone.
By clicking on one of these open jobs, you will open the Job Edit Screen and here you can find a History Tab. Any time you open the job edit screen you can access this History Tab.
This History tab will show you all completed and scheduled work for a customer. You can toggle between Finalized Jobs and Contracts for someone.