First, it is important for us to remember that Satan is not omnipresent—he cannot be in more than one place at a time. Only God is everywhere, and only God knows everything, while Satan must rely on his army of demons to do his bidding.
Can Satan and/or his demons read our minds? No. First Kings 8:39 says that God alone knows every human heart. There is no one else who has that ability. God knows what we will say before we can say it, while the thought is still formulating (Psalm 139:4). Jesus, being God incarnate, exhibited the divine quality of knowing men’s thoughts: “He knew what was in each person” (John 2:25; cf. Matthew 9:4; John 6:64).
The Bible does teach us that Satan is powerful. Likely he was the highest of all the fallen angels, as he was persuasive enough to convince one third of the angels to join him in his rebellion (Revelation 12:4). Even after Satan’s fall, not even Michael the archangel dared to confront him without the Lord’s help (Jude 1:9). Satan is the “the ruler of the kingdom of the air, the spirit who is now at work in those who are disobedient” (Ephesians 2:2b). However, Satan’s power has its limits, and reading our minds seems to be beyond his ability.
It would take omniscience for Satan and his demons to read our minds, which they do not have. God is the only One who can know our thoughts. However, Satan and his demons have been observing and tempting human beings for thousands of years. Surely, they have learned a few things about us over the years. Even without the ability to know our thoughts, they can make a well-educated guess as to what we are thinking and then attempt to use that to their advantage. That is why we are commanded to “submit yourselves, then, to God” (James 4:7a), before we are told to “resist the devil” (James 4:7b).