OOP Snape's Veritaserum

[bubbaqrib]

No: HPFGUIDX 67695

While everyone is contemplating Snape's motive and alliances.  I 
keep coming back to an issue that bugs me. Dumbledore stated that 
Snape had given Umbridge water instead of veritaserum.  How do we 
know his is true since Harry never drank it?  It would have been a 
good way for Snape to betray the Order and let Harry take the fall.  
I personally am not sure Snape gave him water.
Any other thoughts on it.