This newsgroup deals with the Microsoft Dynamics point of sale applications; Retail Management System and MS Point of Sale. You may find more information in one of the developer or language specific groups.
To answer your question, it depends on the scanner. Most scanners work as Keyboard Emulators - they are basically just really fast typists. If you have this kind of devices, your capture is dependent upon the user putting the cursor into the right field before performing the scan. Other scanners have drivers that you can integrate with to capture their input at the device level - the best source for information on this will be the manufacturer's websites.
Good Luck!