If your tanks are not concealed by an enclosed underbelly, just have a look under the trailer. The black and gray water tanks are made of black ABS plastic and the fresh water tank is made of white polysomethingorother.
As jaygiven said, you can look below the pump from the inside. My 23B has about a 4" square hole cut in the floor for the fill hose and suction tube to go through. On a bright sunny day you can see light shining through the tank and up the hole.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.