My understanding is that these (pictured) risers were pressure tested (years ago) and repaired. I MIGHT have raw water leaking into the oil/heads and want to test this component but have not found a shop that knows how to pressure test.
I have inverted the elbow and filled the outer sleeve with water (to see if it leaked into the inner pipe) but have found nothing yet (plus its is only under atmospheric pressure and room temperature)...any suggestions?

Make sure the riser is dry.

Plug the hose barb with a plug suitable for solvents, then stand the riser up on its end and fill the riser with laquer thinner. If there's a crack or pinhole, the solvent will make it through (you may have to watch it for a time).

The above is probably the easiest method without locating an industrial donut seal to seal off the two pipes at the tail end.

I thought you were going to replace the risers with the more aggressive height/drop risers from Hardin/CP Perf?

Thread explaining"

Exh manifold leak test w/acetone

Thanks Tom! I went for a run and came up with the idea of using the garden hose pressure into the port to simulate impeller BUT I like your idea better.
I am for sure going with some sort of more aggressive risers, BUT I want to determine (as best I can) what might have gone wrong. A previous owner had minor water intrusion from the same risers (that he had repaired) and that situation has manifested itself again...then I could resolve it (plus significant margin of safety for the current cams/performance) with a high rise.
However, if the water intruded from another source (oil cooler, end of exhaust reversion, too low idle rpms, too much moisture not burning off) I need to address those issues too (oil thermostat, bump idle, etc...).
I prefer not to go through this again. :-)
A bunch of triple/quad outboards around here are showing off...I need to get out there!

I used a boat drain plug and filed with acetone...no leaks yet. :-( I was hoping to resolve the water intrusion issue.

We did this recently

All you need is a piece of exhaust hose, a fat o ring and some hose clamps.

O ring goes on inner pipe, hose goes over outer pipe and o ring, then tighten clamps.

Put 20 or so PSI air pressure in the water jacket and submerge the whole thing in water (or figure out how to fill the inner pipe) and look for bubbles/