A generally means audio taper and B generally means linear taper. In the example you asked about B10K means a 10K linear taper and A500K means 500K audio taper.
test them out to see if they are indeed log pots.
i.e hook up a multimeter and measure resistance as you rotate pot. See if the resistance increases slowly at the start of the rotation (for constant rotation speed) and increases quickly towards the end of the rotation. Then you have a log pot.