Postive / Negative ground system

[transit360]

I have two Bluebird QBRE's and they are wired a little diffently.
In short, how can I tell if someone has wired one bus or the other
for a positive or negative ground system? Please only advise if you
can supply me with some tests I can run with a meter or if you've
had experience with electrical systems. I'm running inverters and
they've warned NOT to use these with a positive ground system.
The reason I ask is because there is a + and - terminal on the
battery hooked likewise to the alternator. Then I have a 3rd wire,
probably 6 gauge that runs to the starter.
NOW, on the 1st bus this 3rd wire runs from the + on the
alternator to the starter SOLENOID, where is shares a main + feed
from the battery.
On the 2nd bus, this same wire runs from the - side of the
alternator to a ground stud of the back of the starter BODY, where
is shares this stud with a main ground strap. Is this a postive
ground system?
I only discovered this after on the 2nd bus I hooked this
wire to the + side of the alternator to match the other bus, only to
have it heat up. I didn't know why until I traced the wire to the
ground stud on the starter body.
Would anyone know of any reason why a mechanic would do this,
especially on two identical buses? And more importantly, how can I
wire this system up to guarantee that it's the typical negative
ground system that I need? Or is this system still a negative ground
anyways?? I have a little electrical knowledge but I'm stumped and I
really don't want to damage my new inverters.

Thanks,
Steve
Two '98 Bluebird QBRE Partybuses