I have heard so many pastors say, "I'm just not going to tell our [church] plans out loud, cuz I don't want the devil to hear it and interfere." That is ridiculous!
It is said as though we are fighting wars with entities just like us, and that we are afraid that if they know what we are thinking to do, that they will have power over us! And we're going to trick them by keeping them from knowing what we're thinking...
The weapons of our warfare are not CARNAL, but mighty through God to the pulling down of the strongholds.
Every family on earth has 'familiar spirits' [family spirits] who travel down the family lineage until their legal rights are finally broken and they are evicted. That does not mean that you are POSSESSED by the devil. Demonization is a better word. It is the word that is best transliterated from the Greek word used in the NT.
Demons assigned to your family know all your [and your family's] secrets, strengths, generational inquitous bents, generational blessings and your weaknesses. Sit in on deliverances of Christians and you will get a better picture.
The question is not whether 'the devil' can read our minds, but rather what power do we have over him!?? And it's not just ONE DEVIL to worry about, there are many unclean spirits/demons that afflict humans. So, it doesn't matter if he's not omnipresent, he has many helpers and there are enough familiar spirits to attend to all the families on earth.
For example, when we do deliverance, we take the Sword of the Spirit and cut off any demons inside the person from having any communication or gaining any power from any outside source. Sometimes the demon will cry out for help from its buddies - but when we remind it that it's cut off, it acknowledges this in a despairing tone. In the daily course of their business in the lives of humans, they communicate with one another and conspire against us to get us bound in more oppression and curses. The gatekeeper demon tries to bring in others and get us to sin and also give them legal right to be there. [Don't sin! Choose life! Obey God! Stay free! Walk in obedience and the blessing of the Lord will be a crown on your head and your offspring will have that blessing to pass down generation after generation!]
So, instead of worrying about the devil and what he can and cannot do, I would suggest beginning to get a grip on the power of the Word and the authority in the NAME of Jesus Christ. Jesus Christ alone has all power in heaven and earth, and He has given us Power of Attorney to act in His name. He has given us spiritual gifts to defeat the works of the devil in our lives. Walk in that authority and spiritual power! Repent of any wrongdoing, and command any spiritual demonization to leave you. "If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness." We get free by repentence.
Resist the devil and he will flee from you!
It's a moot point, in my opinion, to worry about whether the devil/demons can read your mind. Let's say even if they can't, they know so much about you that they could likely predict what you will do. HOWEVER, they can only try to trip you up, but unless you bow the knee they cannot have power. The point isn't whether we have to fear if they know what we're thinking or saying, but what kind of shield we have around us. The Word in you is a WALL OF FIRE round about you! Make it so hot and furious that the devil will hate being near you.