you can select all in editmode and run 'Recalculate Normals' (CTRLN). OR if they are all the wrong way around: you can simply flip the normals. (Access both from the mesh face menu with Ctrl + F)
This also happens sometimes when exporting to unity give it a try as it may work
PC SPECS: Windows 8 Pro 64-bit, Intel I7 (PASSMARK 9529) 4GHz CPU, Asus R9 3GB GPU (PASSMARK 6858) 32GB DDR3 MEM (PASSMARK 2842)