Assume an appropriate tire patch area.  Distribute load to the tires.  Locate the heavisest load in worst case loacation.  May have to check several locations to verify.  

Note that fork lifts have different front wheel loads (usually much greater) than rear wheels.
Make sure you give yourself enough time.  This is a messy analysis.  You'll have to check punching shear, bending on the composite slab; move the wheels to create maximum shear, check that; move the heaviest loaded wheels directly over the supporting beams and check them.  Depending on the size of the lifting vehicle, you might get contributions from the back wheels on the beam analysis.
Don't assume anything!  Just because the composite slab seems like the weak link doesn't mean that the beam connections, for example, are adequate.

Just a reminder that if you are doing this analysis, get all the loads and data in writing from the contractor. People sometimes have short memories on what they did or did not say.