The devil cannot read our minds! Satan is not omnipresent – he cannot be in more than one place at a time. Only God is is omnipresent, only God knows everything. That is the beauty of our creation, because of that we can relax. The devil can only know things that we speak. He is a tricky guy though and can trick us into speaking our thoughts or intentions, we just have to stay in God's light so we can recognize satan's attempts at manipulation. ~Ev
What Satan can affect, apparently, is one's thought processes through the flesh. He is, for instance, indicted for blinding the minds of the unbelieving (see 2 Corinthians 4:3, 4), and he darkens their understanding (see Ephesians 2:1-3 and 4:17-19). He and his demons can communicate false doctrine (see 2 Corinthians 11:13-15; Galatians 1:8; 1 Timothy 4:1; 1 John 4:1-3), and his demons can impact the thoughts of believers as well. Satan is credited for prompting Ananias to lie to the Holy Spirit (see Acts 5:1-3), for moving David to consider his own strengths when he numbered the people of Israel (see 1 Chronicles 21:1, 2), for inspiring Peter's resistance to Christ's statement about His impending death (see Matthew 16:23), for inspiring worldly wisdom rooted in jealousy and ambition (see James 3:14,15; 4:7), and for leading minds away from devotion to Christ (see 2 Corinthians 11:3). It isn't hard for Satan to know what you're thinking when he gives you the thoughts!
Furthermore, Satan and his demons use deception to give the impression they can read minds and know the future through divination and fortune-telling (see Acts 16:16,17). Satan has also had opportunities since the beginning of creation to observe human behavior. As a result, he has a thorough working knowledge of human behavior. He has learned what he must do to derive certain behaviors from the person he attacks. He can also influence events by influencing key leaders. He uses this deception to give the impression he is like God, able to read minds and control the future. But remember that Christ has triumphed over Satan and, in Christ, you have the responsibility to resist him in submission to God. When you do, he will flee (see Colossians 3:15; James 4:7).