Two ways of knowing. If you hook up a teletype or PC as a dumb terminal go to setup and press edit it will come up with BOSS ? INPUT. If there is data in memory it will come up with BOSS ? EDIT. The ? is the BOSS level determined by the PROM on the ERS board.
The other way would be with the part number on the ERS board.
IE: 1927990 is a BOSS 5 with RS232 as a printed circuit board.
1927424 is a BOSS 4 with RS232, no metric, printed circuit board.
1927106 is a BOSS 4 with out RS232.
Schematics for the PCBs are typically in the Maintenance manual. My scans are too big to attach here.
http://www.machinemanuals.net/products.php?cat=11
Above is a link to a maintenance manual on CDROM for $15.00.
George