Answer:
You really don't. An FCE ihas certain tests built into it to guage whether you are putting forth your maximum effort. You can report to the tester if you are experiencing pain or any other symptom that causes you to want to stop or interrupt the testing in some way but it's better not to try to cheat because they will be testing your pulse rate, blood pressure, and asking you to do various tests and exercises and range of motion movements. I think in a lot of cases it is pretty obvious when someone is not exerting to maximum effort and that will hurt you more than being able to perform some of the requested exercises. Really the best way to handle it is to have your doctor put whatever restrictions on the test which may be appropriate to your situation given your physical impairments and always bring a witness to these evaluations to be sure that everything is reported as honestly and objectively as possible. Once you get the report, your doctor can probably find ways to rebut whatever unreasonable conclusions the report draws. Remember, this is just a snapshot of your ability to perform certain exercises on a given day and doesn't take into account at all how you are affected the next day, week or even later than that.
A well conducted FCE done by an experienced assessor will soon discover if you are attempting to cheat. The FCE is a 'battery' of tests and if you try to manipulate the tests there will inevitably be discrepancies in the results that are obvious to anyone, including your own doctor. Remember that one of the reasons for ordering a FCE is to objectively assess what your work ability is. Try to cheat and you will be found out!