You can start by asking the source who they are, who they work for, how they came to be in Syria, etc. Then try to confirm that information independently. That's what journalists are supposedly trained to do. If the source can't be corroborated, don't use the video. (Just like a journalist wouldn't attribute a quote to someone if it couldn't be verified.) Any reputable news organization would have a fact-checking department that does this kind of stuff.