Assuming you have access to the main breaker for one apartment or the other, turn it off and see if the power is still on in the garage. There's no way to guess.

Who's garage is it?
Who's basement is it?

The wiring is done how ever they want it done when built.
Just because there is two meters does not mean the wiring was divided properly, but does imply that.

Most houses with two meters I have dealt with (and that's a LOT) were not nearly split up 100% properly and legally.

In my area one 60 watt light bulb on the wrong panel can bring the wrath of the utility.

Areas that are 'common' to both apartments, (in other words, if both tenents use them), should be on a
separate meter, called a 'House Meter'. Unless
written agreements have been made with both tenents.

"should be on a separate meter, called a 'House Meter'"

Generating another monthly charge from the utility no matter how much power is used (or not used).

It all depends on the size of the 'common' load.

The house was built for my parents and grandparents and now there is a tenant. I was just wondering who was paying for the garage and basement electric. On which meter it was. There's no house electric bill so it's on either of our bills. Wondering which one though. Thanks!

In that case there is ABSOLUTELY no way of telling short of testing the lines.

"short of testing the lines."

Or just turn off one service and see what happens.