Generally, floor joists run perpindicular to the ridge of the roof, but
this is not always the case. Check the basement, crawl space or attic;
the 2nd floor joists will most likely run the same direction. Or, if
you can determine which way the long edge of the drywall seams runs on
the ceiling below the joists are probably perpindicular to that seam.
Or, try a stud finder on the ceiling below.