A square slab will have two way action. However if you are unsure about locations for openings for future services, or are short on time, you can design as a one way slab. This approach is very conservative.
To design for a two way slab consult a text on plate theory. "Moments and Reactions for Rectangular Plates",W.T. Moody. is pretty good.
You could design it as a two way slab, but it will be a lot more effort for a small benefit. For one, only bars in one direction can be bottom-most. I've done many of these type structures and I design them simple-simple, one-way action and use the same bars in each direction. Make sure that shear and minimum thickness for deflection requirements are satisfied.
I will probably design it as a one-way slab and move on because of the extra effort to design as a two-way slab. This is a very small project and a tight budget. Thanks for the tips!
Since you are not framing into the masonry sides with the concrete top slab, check out the formula in AASHTO Chapter 3 for the loaded slab supported on all four sides.
