Say you’ve found a host lying on a street. Is there any way whatsoever to determine, whether it’s still just a bread, or maybe it’s already transformed into god in person?

It’s possible to determine, which floor was used to bake it, more or less when did it happen, how much smog have settled on it in the meanwhile. If we really try, we can figure out practically it’s whole history – but it’s impossible to say, if it’s entire nature changed drastically? That means just one thing: no, it didn’t change.

If a body A and a body B are totally indistinguishable from each other (whether physically or “spiritually”), then A = B. God’s not in that bread. Transsubstantiation is a myth.