A common problem and can be addressed in a couple of ways. Firstly at the bottom of each SunSystems 5/6 screen, in the right hand corner, amongst other things it displays the current business unit they are assigned.  In addition you can assign ‘colours’ to each of the business units so that all the screens for a business unit are displayed in the same colour. This way you can get a very visual flag as to what business unit a user is working in.
Assigning the business unit colour is done on the business unit setup screen. Depending on the version of Infor SunSystems it is via the function Business Unit Setup for Sun 5 and 6.1 and via Business Unit Administration/Amend for Sun 6.2.