I have a 1978 Mercury 150hp Black Max (requires premixed fuel) that's been parked for a few years.
I've decided to sell it and want to start it up again so that any prospective buyers can hear it run.

First off I've made sure that things are still turning over freely by hand. Poured a bit of oil into each cylinder and turned the fly wheel over many times. Good news... all parts are moving nice and freely.

So next I make sure to avoid using any of the old gas that's still in the tank and bypass straight to a jerry can.

Next part of the plan was simply to hear the engine fire up for a few seconds to say that it will. A shot of mixed fuel into each carb for each cylinder convinced it to start up and it sounded great for the brief moment that I allowed it to run.

Now here's where I'm needing a bit of guidance. My concern is that the carbs may be gummed up and that there's potential that should I run the engine for a period of time long enough to warm it up that maybe one or more of it's cylinders may not get the fuel (and thus the lubrication) required.

How should I be going about testing to tell if the carbs are all working properly so that I don't do any unnecessary damage to my engine when I run it?

take the air breather off start it up you will be able to see fuel go down throats if not plugged and if a float is stuck fuel will over flow hope that helps

I would get it into a body of water... and First besure its pumping water... And then with your jerry-rigged premix... add some sea foam... let it run awhile with that... You could even fog the engine with sea foam...

After i take my boat out of storage every year I always start it and get it running for awhile let the motor come up to operating temps.. and then drain the bowls on the carb.. and pump straight sea foam in the carbs and let it set for a few hours...Then add the premix can and let it run... Sometimes after that it will kill the sprak plugs..