How can I make sure that the ledger board supports for a floor are level?
Use a level. There are many types. 


A simple string and a bubble level. Use a 4-6' level at the center of the span (to account for sag). 
A bubble level on a common joist. Obviously this is heavier, but with a helper it can be done. Set your first ledger and install a hanger. Place the common joist and lay a level on the other end, extending to the wall. 
A laser level. Mount it in a convenient location and measure up from its beam.
A water level. Fill a length of clear tubing with water. Lift both ends until the water line is at the joist height.