Suppose 3 blocks are stacked on top of one another and a constant force is applied on the bottom one, how could I know if the friction that is acting on those objects is kinetic or static? Moreover, how could I extend this method to N blocks? I understand the method with 2 blocks (You assume that they are one block and then compare the acceleration) but I don't know if it applies to the 3 blocks case... For the sake of simplicity, assume that all masses are equal.
Well, I would compare the velocity of each block relative to the other blocks, for the bottom block, I would compare its velocity to the block above it, and if it is moving with respect to the bottom block, then there must be kinetic friction there. The max static friction applied for the top and bottom of block 1 would be different, though, because there is more force acting at the bottom of the first block as opposed to the top of the first block. I would do this same sort of relative analysis with every block, to see if the force applied exceeds the maximum static friction value.