Oops, typo on my part, should read "easy way to tell", not "best way" -- best way is, of course, right at the ground terminal, even with the battery removed. More or less, if it's a known good ground, it becomes quite easy to have that guy there.
Unless of course you like wires, in which case insulated alligator clip to the ground terminal and to a single wire that you connect to your multimeter. Can be a bit of a bear though.
Adding my approach: I'd skip any and all line-level audio output from the radio entirely. Keep it signal level, go to a dedicated amp that connects directly to the battery. Far easier to do this when the battery is so easily accessible to itself and the radio, and when you have the most convenient route for running wires right underneath that ashtray / shifter knob (I've abused the hell out of that one actually). That said, I'm a bit of an audiophile, so I do strive for more clarity in the sound, which necessitates a good amp by default. Or a super expensive radio. But that's just me.
Grounding wire should match for the radio. In and out ideally matches. There's an electrical reason for this, I'm not well versed in the area where wire width and length come into play, but the Daves on this forum are and should be able to clarify that one.