That depends on how you want to present this to your users.
You can either check PRIOR to opening the form (make all records 'Locked' blue or something) AND prevent the form from being opened in edit mode OR change the mode when they are moving through the records on your form(s).
It depends on how you are displaying data and how you are allowing users to select records. If you only allow them to view the data in data sheet mode prior to selecting a record for edit, THIS
thread can help. If you are allowing a different way to select records, you will have to work out for each approach how you want to notify the user they can't edit data.