* I don't know how to check if the transmission face is parallel to the rear of the block.
* I don't know if the 2 faces of the BH are parallel or if the hole is concentric. I don't know how to determine this.
You do both the same way, in a matter of speaking. This is best done at assembly, with the engine vertical. The thing is CRANK END PLAY (thrust bearing) will affect your readings---so far as transmission face parallel to the block.
Basically you fasten the indicator device to the crank or flywheel, so it rotates the feeler / indicator on the bell face, so that as you turn the crank, the indicator rotates around the mating face area of the bell.
Another way would be to assume that the block is OK, and set the bell up in a mill or some other setup where you can establish precision. Basically all you need here is to see if the transmission face is parallel to the block face. You could put it bell down on a mill, attach an indicator to the spindle, and move everything around to determine the "4 corners" of the transmission face are at same height from the table.
Same thing "concentric hole" You simply indicate on the inside bore of the bell hole, and rotate the crank to see what happens at the indicator.
By the way if the block has been line bored (align bored) the centerline could very well be off. I am CONVINCED after all these years, that my old 70 440-6 car was misaligned......it was fairly hard shifting and "used up" pilot bushings.