Often it is difficult or impossible to evaluate validity of witness statements, but there are exceptions.
Pay close attention to witness statements: are there actual confirmations to the journalistic claims made, or are they speaking on some general subject matter (which the mass media would later use in their own interests)? For example, in the above-mentioned story with the yacht, NTV channel personnel also tell of another businessman (a Donetsk flight instructor), who donated a plane to the militants. As it turns out, the instructor was actually answering questions on airplanes and skydiving, and had not donated anything to anyone. Later, he even recorded a video-address on the matter.
Another such example is a story by news channel Russia Today, which tells of Jewish people fleeing Kiev due to anti-Semitism of the new Ukrainian government. The channel’s personnel even showed how Rabbi Mihail Kapustin is packing and saying that he’s forced to escape the oncoming danger. However, if you listen to his words, you will discover that he somehow doesn’t say which side he’s afraid of, exactly. If you dig deeper, you will further discover that he’s not a rabbi of a Kiev synagogue, but a Simferopol synagogue; he had earlier publicly come out asking for defence of both Ukraine and Crimea from Russia. Now, he is fleeing Crimea due to the new Russian government.
Furthermore, pay attention to the actual witness: have you seen him/her before? For example, the famous “travelling performer” Maria Tsipko regularly gives witness statements to mass media, acting as a resident of Odessa, Donbass, or any other hot spot.