Slab rebar callout:
Top Steel: 9-#4 longitudinal, 4-#5 transverse
Bottom Steel: 4-#5 etc., etc.
Or 9-#4 longitudinal, 4-#5 transverse, top and bottom
With or without leaders pointing to the reinf.
There's other ways, too. Anything that communicates to the rod-busters.
Bob