Measuring force developed by a robot
I've always used a standard spring kitchen scale rotated. Put the scale between the robot and a wall (or another fixed object) and let the robot push... 

BTW: This is one of the tips that are also suggested in the robotic sumo chapter of "Building Robots With Lego Mindstorms" book by Mario and Giulio Ferrari (two italian RCX gurus). So, it's really "in topic" with the original question.