That's rather surprising about both pins 15 and 1 going to ground. You need to regulate the current in each winding independently, so if these pins are both connected to ground this means they are connected together which in turn means you can't obtain separate readings for their current's. Putting resistors from pin 1 and 15 to ground is basically the only way to do it as that's the only 'available' point in the circuit. The voltage across these resistors is then amplified within the L297 and most likely using a comparator with voltage reference is used to chop the current. This is the method I have used, except with a PIC and substituting the fixed voltage reference for a DAC to enable microstepping.