They are checked during the engine run-up procedure. The fuel system should be set for the takeoff. If possible, position the aircraft WELL TO THE SIDE of the taxiway or run-up area, so as to permit other aircraft to pass. These numbers should include at least Vx, Vy, and the best glide speeds of the airplane. There is nothing wrong with also noting the appropriate takeoff and landing distance requirements for the airplane under the existing density altitude conditions.
Nothing is wrong with turning a complete circle, if that's what is needed.
High-speed turns are one of the leading causes for landing accidents. Airplanes are very top-heavy, so they tip easily. If the student has shown proficiency in being able to accomplish this procedure while taxiing, the aircraft need not be stopped. Prior to engine shut-down, call the company dispatcher to advise them of your return. If they are not, the slightest movement of the propeller may cause the spark plugs to fire, kicking the engine over.