If there is a waterlogged stair upside down and there is a transparent block below it, the water cannot be seen from below (as seen in picture). Is non-directional from my testing.

How to reproduce:

1. Place stair in such a way that there is an "empty" area facing down.
2. Place a transparant block, such as a barrier or glass, 1 block below the stair (no air gap).
3.  [Optional] For a cleaner setup, surround the blocks around the stair so that the water doesn't flow out when you do step 4.
4. Waterlog the stair.

Observed results: 

From beneath, it looks like the stair is not waterlogged.

Expected results:

From beneath, it looks like the stair is waterlogged.