Sounds simple but tough to put in practice.

 

John 10:34-38(NKJV)

Jesus answered them, “Is it not written in your law, ‘I said, “You are gods”’? 

If He called them gods, to whom the word of God came (and the Scripture cannot be broken), 

do you say of Him whom the Father sanctified and sent into the world, ‘You are blaspheming,’ because I said, ‘I am the Son of God’? 

If I do not do the works of My Father, do not believe Me; 

but if I do, though you do not believe Me, believe the works, that you may know and believe that the Father is in Me, and I in Him.” 

This can be a bit of a slippery slope because we are told the beast or the anti christ will work what appears to be miracles just like Jesus so many will believe in the anti christ.  How do we tell the difference?

Easy.  Do they agree that Jesus is who He says He is or do they deny that Jesus is the Son of God.  If you see them someone who seems to be "doing the works of the father" and yet denies the divinity of Jesus then you can be sure that they are not of God.