Priest Gremory:
Go to the Room of Splendor and pour a measure of water into the Holy Grail. Then pour the water back into a jar and bring it to me. We can test the water to determine if the Grail is the real thing or not. Be careful when you do this; the room may be guarded.
Oh, another thing. The Room of Splendor is locked, and you'll need a key to enter it. Some of the monks carry a key to that room; I suggest that you overcome of one of them and take his key.