Thanks for your questions. I’m a little pressed for time at the moment and unlikely to be online for a few days, but I’ll answer the best way I can with the time I have atm. Christianity is such a vast topic which one could study for a lifetime and still not fully know everything, especially where mysteries are concerned.
When (and how) does the wafer and the wine become the actual flesh and blood of Jesus?
When - at the words of Our Lord, ‘Take and eat, this is my body’, ‘Drink from it, all of you, for this is my blood of the covenant, which will be shed on behalf of many for the forgiveness of sins.’
How is much more difficult to explain, as it is with all of Christ’s other miracles. The nearest I can think to explain it is with how Christ humbled Himself by becoming man.
How do we know that it has become the actual body and blood of Jesus?
Because Christ told us. John 6:53-56 Jesus said to them, "Amen, amen, I say to you, unless you eat the flesh of the Son of Man and drink his blood, you do not have life within you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise him on the last day. For my flesh is true food, and my blood is true drink. Whoever eats my flesh and drinks my blood remains in me and I in him.
How do we know that any of this ever happens during mass?
Christ told us to do this in memory of Him. This is not simply recollecting… the first Christians were Jews and this is not how the Jews understood it. It’s basically a fulfillment of the Passover, and Christ is our Paschal Lamb. I realise this is complex and not easy to explain in a few simple words. Similarly, the manna of the Old Testament is a foreshadowing of Christ’s Body and Blood.
How can we know that the mass is a sacrifice rather than a way of remembering the sacrifice of Christ?
Besides Our Lord’s words, St Paul tells us, too:
1 Corinthians 16 ‘The cup of blessing that we bless, is it not a participation in the blood of Christ? The bread that we break, is it not a participation in the body of Christ?’ and
1 Corinthians 11:26-28 'A person should examine himself, and so eat the bread and drink the cup. For anyone who eats and drinks without discerning the body, eats and drinks judgment on himself.
It would be helpful for you to go back to the beginning of Christianity… see how the early Church understood these topics. The Church has a very rich history. Anyway I’m out of time.