1997 NG900 2.0 turbo.

I have a moderate (lose 1/2 liter/month) water leak from the pump or
the seal into the engine. It leaks enough that there will be a large
damp, not wet, spot on the driveway in the morning. I can see where it
drips down onto the oil filter area from above by looking at the
bottom of the engine. I can't see anything from the top as there is
little viewing room.

I did change the pump last year and I did a very diligent job of
cleaning the iron block entrance with #400 sandpaper until it was
smooth and shiny again and carefully fitting the gasket to the pump,
etc (bearings went on last one).

Any ideas on how to pin down the source?  You can't see under the pump
from the bottom or the top. I suppose I could just change the pump
again, but if it is only the block ring gasket I could save a lot '
of $'s. Of course, it's not a quick job and I really don't want to
do it twice more (i.e. change the block gasket and then have to do
the whole job over again to change the pump if it still leaks).