There’s magnet viewing screens that let you “see” the magnetic field.
But I find a cheap test is if you have several magnets: the radially magnetised ones like to stick to each other side to side, in a “chain”, while the axially magnetised ones (wrong kind!) stick face to face, in a “cylinder”…
I had the BGC 3.0 board working quite well. I would start out getting only 1 motor working to begin with. You will find that the combination ATMega328P and I2C sensors is not very fast - but it will turn two motors (just barely) with some optimisations to the sensor code. Don’t expect to run a lot of other code on this MCU as well though…
As to why it is not working, could you give some more details on your code/setup? Its a bit hard to say what the problem is from the information given, but if open-loop is working, and sensor-reading is working, then you can’t be too far from closed loop control!