From cin.org/users/james/files/praying.htm :
Q: How can the saints hear our prayers? Aren’t you making them out to be omniscient and omnipresent?
A: Certainly not! That is a canard often tossed out by anti-Catholics who are not acquainted with the Catholic view of the saints.
The saints certainly have more knowledge than we do in this life:
“For [now] we know in part … but when perfection comes, the imperfect disappears. When I was a child, I talked like a child, I thought like a child, I reasoned like a child. When I became a man, I put childish ways behind me. Now we see but a poor reflection as in a mirror; then we shall see face to face. Now I know in part; then I shall know fully, even as I am fully known” (1 Corinthians 13:9-12).
We will never be omniscient, and certainly we will not be omnipresent, but we will be aware of many things which we are not now cognizant of.
The standard account for how the saints are aware of our prayers is that, because they have the beatific vision of God, they see in God all of the knowledge they need, all of the knowledge that is relevant to them, and so they see our prayers to them. On the standard account is thus by the omniscience of God that they become aware of our prayers, though they themselves are never omniscient and never take in the full scope of God’s knowledge, only those parts that are relevant to them.
However that may be, there is simply a big, huge difference-in fact, an infinitely huge difference-between being “multiscient” (knowing many things) and “omniscient” (knowing all things). We will never cross from the one to the other, and it is simply a straw man of anti-Catholic posturing to represent the expanded knowledge of those in heaven as if it were infinite knowledge. It is a classic case of triumphalistically bashing a position that nobody holds.