One generally tries to determine the properties of the plasma without perturbing it by investigating the effects exerted by the plasma on the outside.
These are magnetic or electric fields, charged or neutral particles, and electromagnetic waves which the plasma emits in a wide spectral range from the radiofrequency to the X-ray regions.
Besides these passive methods, active methods are also applied insofar as one can be sure that they do not change the plasma.
A particularly rewarding technique is to beam in laser light or microwaves, which can be influenced by the plasma and thus provide information on its properties. Particle beams are also used for diagnostics.
Systematic measuring errors are avoided by consistently applying different methods to measure the same physical parameter.