Kim,
JVN's 2008 spreadsheet (found in the white pages) does that exact calculation for you. He compares the maximum torque from the gearbox and chain reduction to the actual frictional force from the wheels, and uses the lower of the two numbers to determine your maximum pushing force. The spreadsheet even has the ability to combine multiple motors.
The difficulty is determining the Co of Friction for the various wheels, surfaces, etc... That's where FRC842's bathroom scale and immovable object help out.
Good luck,
Bengineer