Mountain folds will look like a mountain (folded up) if your looking at the sides with the numbers.
Valley folds look like a valley (folded down) when looking at the sides with the numbers.
Of course, it's all relative to the project you're doing, the way it was printed, the settings in the program, the way the model was unfolded, and a lot of other factors.
But in the end, just locate the piece on your model and see which way the fold goes. I still can't believe there are so many people that ask this...