How do we know who is and is not saved by Jesus's act on the cross?
Judas in particular was not. Matthew 26:24
The Son of Man will go just as it is written about him. But woe to that man who betrays the Son of Man! It would be better for him if he had not been born."
As for all others:
Matthew 12:32
"Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come."
Notice that the act of speaking against the Holy Spirit requires that one know and reject the holdings and values of the Holy Spirit, not simply that they are ignorant of them. "Father forgive them, for they know not what they do." applies to the slow and ignorant, not to those that knowingly reject the Holy Spirit. God taught you to pray, "give us this day our daily bread" and "forgive us our trespasses, as we forgive those that tresspass against us". Bread here is not viddles, it's the Holy Spirit. John 6:32-35
Jesus said to them, "I tell you the truth, it is not Moses who has given you the bread from heaven, but it is my Father who gives you the true bread from heaven. For the bread of God is he who comes down from heaven and gives life to the world."
"Sir," they said, "from now on give us this bread."
Then Jesus declared, "I am the bread of life. He who comes to me will never go hungry, and he who believes in me will never be thirsty.