Would the allegation that someone is a "spy" pretty much be a no win situation for the accused?

Let's say that spying is a covert activity. And let's say that the nature of the covert activity is meant to be so that observers cannot directly know whether he/she may be one or not. Would this mean that if someoen is accused of "spying", he/she cannot get out and defend himself/herself, unless people use trial by ordeal tactics (a no win situation for teh victim) in order to try and "extract evidence"? And what if the people doing teh investigation is so biased and intention of "finding evidence", that he/she needs to create one? What happens in this case? I'm not talking about 1950's McCarthyism but a modern day scenario. How does this situation get sorted out?