Charger & Load Detection with MSP430G2553

The normal way to do it would be measuring voltages and decide what to do.

In your setup, this is a bit tricky, since you have different GND levels (the FETs are separating the GND into different segments.
Since the G2553 can only measure positive voltages based on its own GND/VSS level, this could be tricky.

For further suggestions, a bit more detail is required than this rather rough schematics.

Gate of CHG & DSG FEt is controlled using MCU by measuring cell voltages. If cell voltage falls below a desired threshold then gate of DSG FEt (n- channel) is grounded so that VGS in DSG fet reduces to 0V. This opens DSG FEt & o/p voltage is zero volt. Similarly if cell voltage rises above particular threshold then gate of CHG fet is turned off. This is basically to protect cell from upper & lower voltage i.e to keep cells in safe operating area. However when a charger or load is connected, this condition has to be tested & at this point both fets are to be turned on.

So MSP is powered by BATT, BATT+ being VCC and BATT- being VSS?
Then simply measure VCC/2 through internal ADC channel, and control the FETs as desired.

If MSPs VCC is from the pack, then you need to measure BATT+ and BATT- to get the difference voltage. And controlling the FETs with their relative drain voltages can be difficult.

So what and how to do it depeds on 'the whole thing' and that's why I asked for more details.