determining current in individual phase for a 3 phase winding brushless dc motor
The absolute best way to measure the current in a line is to use a current probe such as the TCP0030A.  They are a bit pricey, but totally worth it if you are doing motor control.

Of course, if you are doing this in an application, you can use three ACS711 (pdf warning) chips (or similar).  These are good enough to use for current control loops or similar, but not necessarily good enough for lab-grade measurements.

I have used both successfully.

What happens in a stall condition depends entirely on the drive architecture.  Most drives have some sort of stall protection built-in so that they don't burn up the motor.