We cannot, but God can and does. The miracle of transubstantiation takes place on a daily basis and billions have witnessed it.
This underscores a significant difference between Catholics and Protestants. Catholics believe in miracles and witness them frequently. For Protestants and the other odd assortment of Christian-like groups who populate this forum, miracles are something rare and only experienced by people long dead.
Nobody has witnessed anything. It's still in appearance the little white wafer that was put into the chalice. Not raw flesh, no real blood.
All that's happened is that millions have watched, perhaps on a daily basis, priests CLAIM to have changed a little white wheat wafer into the literal body and blood of Christ.
Since no change in physical appearance has occurred, no one can legitimately claim to have witnessed it All they can claim to have seen is a man going through the motions of claiming to have done it.
Besides, since Catholics never drank of the cup, there's no way for anyone to be able to attest that the wine was changed to blood.
Why doesn't the Catholic church serve BOTH elements of communion to its adherents as Christ did with His disciples and commanded us to take both the bread and the cup?
Why the disobedience to the command of Jesus Himself?
Now there's a problem...No one witnesses it...
Let's try something...Here it goes...I'll pretend my old truck will turn into a Cadillac Escalade...
There, it's done...Do you see it??? Ain't it pretty...It's fast too...Want a ride???
There's your transubstantiation...