The force is obvious!
As a pedant I would like to point out that the counter rotation of the helicopter is "evidence" that the equal and opposite force exists. It is not the force its self.
I don't see the need for this "evidence".
If you start at the beginning, and consider the function of the engine, then it is clear that the engine produces a force *between*, let me repeat that, *between*, the rotors and the rest of the machine.
No evidence is required. It is the case by definition. Forces *always* have to be between two thingies. No thingies, no force. Thingies may be resistancies (e.g rotor blades, propellors, brakes), or [inertial] masses or some combination of the two. Note - inertial added for emphasis, 'cos there is no other kind of mass. Well not that I know of:-)
There is of course a case whereby a force can be generated by expelling a reaction mass, as in a rocket, from the rotor tip. No current helicopters, however, operate in this way. (Check out Fairey Rotodyne for a reaction helicopter, note the lack of tail rotor, or for the pedants
other opposing force generators.)
I am sorry if I appear unduly pedantic, however this is fundamental stuff and I have the idea that I have a good understanding of it. Sadly I can't tell you how to develop such an understanding other than to keep asking questions.
PS. I am making all this up as I go along, but I think that I am doing OK. Let me know if you disagree.