Install a fume detector, remove the cover over the gas tank or sending unit, check the hoses, fuel pump and carb or FI. But most important of all
SNIFF THE ENGINE COMPARTMENT BEFORE STARTING....
I rarely if ever see people with gas inboards doing that. I once asked the gas attendants at a couple of gas docks about this and they had no idea. None. So I gave them a little safety lesson and said, 'if the captn doesn't check for fumes, you might want to step as far away as you can when he starts it up'......