I've been chasing the source of this red fluid, my boat has a diesely smell especially after it's been run... but is there any definitive way to tell if it's diesel or ATF?
The fluid in the velvet drives is red.
What color is your coolant? It would have it's own stench and taste also.
You could put some UV dye in the velvet drives, first one, then after a run and check with black light, dye the other and repeat. If no sign of glow under black light you can assume it’s the Diesel.
They smell entirely different. So as High Wire said, soak it up with a paper towel. Then get off the boat and smell it. If the boat has a pronounced diesel smell you may have to stick it in a sealed glass jar and take a sniff after you are away from the boat for a bit. I know for me that after I get a nose full of diesel fumes, that is all I can smell for a long time.
Not coolant, I know that taste and it's green.