How do you determine redundant forces in a statically indeterminate structure?

Redundant forces are chosen so that the structure is stable and statically determinate when you remove these forces. So, if you have two degrees of indeterminancy, you will have to remove two forces, remove three for three degrees, and so forth. Redundant forces are usually found when you have reaction forces AND a displacement, as it is obvious one caused the other. So when you remove these forces, the displacements are still there, and the structure has not changed, except that it is now statically determinant, and you can use method of superposition to figure out all your unknowns :)