One of the most obvious proofs that Transubstantiation is not true is that it quite visibly is not true. Every Sunday, when Catholics partake of the Eucharist, they eat bread and wine, and it looks like bread and wine, and it tastes like bread and wine, and it acts like bread and wine, because it is bread and wine.
Because the bread and wine is bread and wine, it is certainly not the flesh and blood of Jesus Christ. And Catholics, of course, know it. By their very senses, they detect that their religion is a lie. They can easily distinguish between food and flesh. Many of them have likely had a taste of both. If they've ever cut their finger, they may have sucked on it to stop the bleeding, and they know the wine tastes nothing like their blood. Perhaps, if only as children, they have eaten their own dead skin. Don't judge them, if they think it's even remotely ok to literally eat Jesus' flesh, why wouldn't they be ok with eating their own? But if they've eaten their own, they know just as much that they're not eating flesh on a Sunday.
"But it's a sacred mystery", they reply. Yes, it's a mystery why anyone would go to such lengths to maintain their cannibalistic fantasies. I have even heard some claim that the reason God doesn't make the transformation visible is because it would scare people away from consuming it. But this is silly for multiple reasons, the first being if you truly believed doing this is necessary to be saved, and there you have the miraculous proof right before your eyes, you're more likely to consume it. If you're the kind of person who can pretend, against all evidence, that bread and wine are flesh and blood, you're obviously going to accept visible proof that you're right. Furthermore, after doing it every week for a while, you'll become desensitised to it, just as those who eat bugs do.
Of course, there are claims of Eucharistic miracles. That is, sometimes, something weird will happen that makes it look like the host genuinely do become flesh and blood. But my simple challenge to those who claim these are actual miracles is put your money where your mouths are. You believe that upon consecration, the host become the actual flesh and blood of Christ. You believe that you must consume it to atone for your sins, you believe you've been doing this every Sunday since you became Catholic, it should be no problem for you to do so. Go ahead, show me your faith by your works.
But of course, they won't. Even if it isn't intentionally fraudulent, there is a high probability of a natural, most likely dangerous alternative explanation. Fungus seems to be the most common example. You expect me to throw out my senses because some numpty forgot to wash out the cup before serving it up to a congregation?
No matter how much Catholics may insist Transubstantiation is a true doctrine, every instinct they have tells them otherwise. They may support it with spurious claims of miracles, they may excuse it with ad hoc excuses for why it doesn't look like it's true, they may even call us stubborn for following the evidence where it indisputably leads, but no case can be made for the Catholic Church's claim that anyone who has ever lived has taken a single bite or sip of Jesus. For sake of both their credibility with man, and their relationship with God, Catholics need to trust the senses with which they were blessed and acknowledge that this strange doctrine did not come from God.