This is a tip that cannot be stressed too much in a retail business, give every user in your POS Software a unique ID and make them use it. Every person should have their own ID. Do not allow two people to share the same login. This will also enable you to monitor, audit and if required you can, then investigate what is happening.
The reason this came up today was because we have just had one client, whose evidence from their system was rejected as the employee in question used a general login and the judge stated it could have been anyone, so it proves nothing.