When you're questioning the witnesses, be very thorough with them. When you're looking at them, know what to ask and how to ask it. Go through every point; don't rush to judgment. If there are false witnesses here, the more information you ask them for, the more they're going to provide, and the more you'll be able to detect if one side of the story is contradictory to the other. You'll have the ability to see if they're lying or not, and if you should dismiss their testimony.
Be very careful with what words you're using to question them. You don't want to give too much information. You don't want to frame the case a certain way and give them a petach, give them an opening to take that they're going to get off. Be very careful when you ask questions; don't give any clues to what you're looking for. Be very careful how you phrase everything, because you want to make sure they don't learn from your words how to lie and get out of the problem.