How do I figure this out?
Do you have access to the plywood subfloor? If you do you can tell by the nailing pattern.
Generally the joist run the with the width of the house not the length.
Now thats generally speaking. Not a 100% all the time fact.
Remember that the joists are usually 16" apart so if you find nails closer than 16", they're probably parallel with the joists while the one 16" apart will be perpendicular to the joists.
I have seen maybe one home where the joist spacing was 24".
If you have heat registers in the floor, pull off the cover and see which way the ducting runs. Typically it will run along the joist bay.