Dad has about 100 gallons of gas in his boat.

Problem is that he put it in over one year ago...he did add stabilizer though.

So is the gas OK or does he need to have it pumped out? Is there a test we could do?

Fire it up! Seriously, he put stablizer in it right? Is it dry stored ( on a trailer or a bunk) or was it docked? Iffin it were me I'd run it down and before adding more fuel I'd change the fuel filters and check the water separator (if it's a diesel).

Good luck!

Yeah run it. Stabilzer works great. My neighbor puts it in all his lawn equipment and generators each winter but make sure you run it a bit to get the stablizer into the carbs.....Ask me how I know......