We see in Second Corinthians that the false apostles did signs and wonders. Paul said they were being deceived by the angels of light. They were in their church. We see another thing the false brothers were doing. They were exploiting the church for money. They also dominated the church, and there were several of them. So my question is, are miracles the way to tell if they are from God? The Roman Catholic brothers and sisters have miracles also. They have bleeding statues, bleeding hands, and oil dripping from their hands too.
I would say that if the individual doing the miracles is in line with other principles in the Bible, then yes. Taken apart from those same principles, no.
The way to do that in my opinion is to look at the miracle worker and ask if there is even a chance that God is working through that person, even though I see something in front of me that is outwardly impressive? 1 John gives different tests for knowing whether we are in the faith, and in part I think it is to help weed out false ministers. Part of the intention was to weed out the immoral gnostics who claimed to have special revelations about God and Christ all the while they lived filthy and immoral lives. The basis of 1 John is character alone, and each time one of these is missing, John indicates that the individual does not know God. Not that the man is perfect in these, but these should at least be reasonable descriptions of the life of that person.
In short, the tests:
1. Walks in the light as Christ did (which I take to mean godliness is his true priority)
2. Keeps God's commandments
3. Is separate from the world and does not love the world
4. Loves his brothers and does not hate
5. Does not continue to practice sin, and practices righteousness instead
6. Acknowledges Jesus as being Christ, with that acknowledgment being expressed through obedience
Notice John said nothing about miracles in relation to their state.
I would rather bring it first to the question, "Can we even reasonably assume the person is a believer"* before we start talking about the miracles he does. One whom you cannot say that these things are true about--his life is filled with hatred, lives in defiance of God's commands, entangled with the world, winks at sin and even scorns righteousness--there's a great chance that he does not know God, even if he claims to have incredible power (Simon Magus, Balaam). From there, you can make a conclusion about the miracles they do (or claim to do), or avoid discussion on them altogether. Even the devil will be able to call down fire from heaven, though none who could see through his masquerade of light would dare endorse him.
Miracles were said to have happened under the ministry of John Wesley. I am far more inclined to believe that the ones under his ministry were genuine because of the man's reputation for godliness. Some ministers claim miracles who have no real godliness; these, I am less inclined to believe.
*I am NOT saying to draw a precise conclusion on his salvific state; I am saying to ask if we we have sufficient reason to believe that he is in good state; is it likely, or doubtful?