So today I tightened up my alternator belt as it was super loose and I'm charging up my battery tonight and will try to crank her up tomorrow. My question is about something my (sometimes) crazy uncle told me about this engine (1992 SNOB 5.8 Pro Boss): Once the engine is running, take the ground off the battery and see if the engine still runs. If it does, then alternator is good; if it doesn't, the alternator is bad. I've seen things online that this can fry the alternator but it seems like that's only on newer cars with voltage regulators? Can I do this test on my engine? Only asking because I don't have a multimeter! I plan to buy one in the future though.

Thanks guys,

Alex

Don't try it. Your uncle is probably old and goes back to the days of generators rather than alternators on engines. 

Get a VOM. A basic meter can be had for $20 or less. 

You could mention to your uncle or yourself that your boat just might have one of those alternators complete with a voltage regulator like newer cars 

The voltage transient can turn a good alternator into a bad one kinda quickly.

It might work, but why take the chance.

What's your dash gauge read if you turn the key from OFF to the RUN position?

What's your dash gauge read when the engine is running?

Compare the 2 readings, that'll give you an indication of whether it's doing any charging

As far as the multimeter, since you plan to buy one in the future, the future should be now 

Plus you can use it in the house and on the car and or truck too. 

Brought the boat to a mechanic since it hasn't had a general lookup since I purchased the boat last month:

Mechanic put in a new impeller kit as well as a new starter & solenoid (must've been the reason why it wasn't starting since I tried to start the boat while charging the battery overnight and only got "click" "click").

Took the boat right from the mechanic to the upholstery shop to get all new cushions (pics to come!), so I won't be able to test the alternator until I get the boat back in ~3 weeks.

The starter & solenoid does make sense to me though because when the engine did start, volt meter on dash was reading between 13 - 13.5 volts, which seems right to me if I'm not mistaken.