Miracle Workers.

On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’  And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’ Matthew 7:22

Then that lawless one will be revealed whom the Lord will slay with the breath of His mouth and bring to an end by the appearance of His coming. that is, the one whose coming is in accord with the activity of Satan, with all power and signs and false wonders,  and with all the deception of wickedness for those who perish, because they did not receive the love of the truth so as to be saved."
2 Thessalonians 2:8-10

How can we tell if miracles or mighty works are from God or Satan?
These first two verses tell us these demons are lawless. In other words, they do not keep the law of God. 
If you see a man, woman, or an angelic being work miracles and many people are following him, you can know if they are from God if they follow the law of God. 

If these people say wars and killing are needed, if they say we must punish those who don't believe as they do, if they have no love in their hearts, if they indulge in sexual sin and greed for money and possessions, then we can know they are not from God.

Jesus told us, “Beware of false prophets, who come to you in sheep’s clothing but inwardly are ravenous wolves.You will recognize them by their fruits. Are grapes gathered from thorn bushes, or figs from thistles? Matthew 7:15

Fruits: "But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control; against such things there is no law." Galatians 5:22,23

If they do not have these fruits in their lives then they are false prophets or messengers.

"Now the Spirit expressly says that in later times some will depart from the faith by devoting themselves to deceitful spirits and teachings of demons..."  1 Timothy 4:1

"But realize this, that in the last days difficult times will come. For people will be lovers of self, lovers of money, proud, arrogant, abusive, disobedient to their parents, ungrateful, unholy, unloving, irreconcilable, malicious gossips, without self-control, brutal, haters of good, treacherous, reckless, conceited, lovers of pleasure rather than lovers of God, holding to a form of godliness, although they have denied its power; Avoid such men as these." 2 Timothy 3:1-5

So, these miracle workers will have a "form" of godliness but do not have God's power in their lives. They say they are Christians, but they are not.

The largest deception will be when Satan transforms himself into an angel of light. 

"And no wonder, for even Satan disguises himself as an angel of light. Therefore it is not surprising if his servants also disguise themselves as servants of righteousness, whose end will be according to their deeds." 2 Corinthians 11:14, 15

Jesus also said, "For false Christs and false prophets will arise and will show great signs and wonders, so as to mislead, if possible, even the elect. See, I have told you beforehand." Matthew 24:24, 25

Jesus said, "See, I've told you ahead of time." We have his warning and his method on how to recognize his true followers.