I just removed the port side riser on my 07 496 Mag. I inspected the hose ports and shined a light inside the manifold and all ports. I see no sign of corrosion anywhere. Is there anything else I should check? Gonna replace the gaskets and reassemble.

since they are dry joint, you can pressure test the manifolds with a hose. cap off hoses to risers, run hose into inlet. look inside for leaks.

so if I have the riser off, put a garden hose on the manifold inlet and cap the outlet?

anyone, anyone, Bueller, Bueller......

Yes.

Or fill with 30psi of air and drop in a tub of water and look for bubbles.

Run a 1" hose jumper between both water outlets on the top of the manifold. Secure with breeze bands, plumb water into the manifold input on the bottom and crank water on.