So is it like faith, we never can be sure who has it and only God knows the truth?
We need to test, which is why I keep banging on about the "test all things" line.
Jesus said we would know people by their fruits. On the fact of it, the occult healing I described could have been presented as a "gift of healing" but if you looked at my life at the time you wouldn't have seen much in the way of the fruit of the Spirit. If you'd asked me where the power came from I wouldn't have given any glory to God.
In Deut 13 we see a warning of false prophets who will come. There are two stages to the test - one is whether the prophecy comes to pass at all, and the second is whether the "prophet" then says "let us go after other gods". It's all well and good displaying signs and wonders and giving out "words of prophecy", but if the word doesn't come to pass we can conclude it wasn't from God, and even if it does come to pass we can then look to see whether the person points people towards God or not.
One thing I've seen quite a lot in various discussions (online and offline) is a challenge where people present some miracle or other and ask whether it's genuine or not. They obviously expect a "yes" or "no" answer. I file a lot of these things under "I don't know", and oftentimes add "and frankly I don't care". If I hear a rumor that Pastor Joe Blow has seen medically verified healings of cancer, the lame walking, the blind seeing, whatever else, I can't verify the medical records. I can't tell if the "lame walking" were genuinely healed, placed stooges, or a crowd conned into believing they were healed because they took a few faltering steps out of a wheelchair. And the chances it won't make any difference to my life at all whether it was a genuine healing, faked with a stooge, or faked based on crowd manipulation.
... which leads me back to another hobby-horse of mine, the difference between "Joe Blow Ministries International" and the day-to-day business of loving people the way Jesus told us to. If I see Pastor Joe Blow on one of his whistle-stop tours through my town he may be a great speaker, he may be truly inspiring, he may be highly anointed by God, but the chances are I'm not going to see enough of him to know. I won't get to see the fruit of his life, I won't get to see the way he talks to his family, I won't get to see how he handles the situation when someone calls on him for help when it's a really bad time, all I'll get to see is a stage presence that could be heavily stage-managed. If I see Pastor Joe Public serving at the church down the road I can see how he talks to his family because I run into him at the grocery store. I can see how he handles awkward situations because the chances are sooner or later I'll be the guy behind him at the checkout when the till stops working, when the assistant can't cope with the coupons he's got, when the person packing the bags puts the tins of soup on top of the soft fruit. I can see whether he lives an opulent lifestyle while constantly pleading for people to give more. I can see whether he truly has time for the needy and the unlovely or whether he palms them off on the Assistant Pastor For Things Beneath Me.
I've come across a few teachers who claim signs and wonders but, having read their teachings, I honestly don't recognise the god they describe. So on that basis I would conclude they are false prophets and false teachers, and regard anything theological they say as being suspect. They may sometimes say something good, but then a broken clock is right twice a day.