Take this for whatever you will. If you're on the Eagles sideline this might constitute indisputable video evidence, if you're on the Redskins sideline maybe more of a funny coincidence. According to My Fox Philly, there is video proof that LaRon Landry was in spitting distance of Mike McGlynn during the game, the player who accused Landry of spitting in his face.
But video from the second extra point shows Landry (wearing no. 30 in red) lined up opposite the Eagles left tackle and walking right up to McGlynn (wearing no. 77 in green) after the kick when McGlynn was prone of the ground.
Landry can be seen leaning over and staring directly at McGlynn, with a distance of just several feet separating the heads of the two players.
The article is quick to point out that there is no evidence that Landry actually spit on McGlynn during these encounters, but rather that he was actually close enough to do so if he chose.
If they went over the video with a fine tooth comb and couldn't really find any evidence of spitting, I'm not sure the league will be able to turn anything up in their investigation. Keep in mind it was raining pretty heavily throughout the game, and it is possible that an over-sized rain drop could be mistaken for a loogie if it hits him in the face while Landry is yelling at him.