Believers--How do you know "Ghosts" are really Ghosts?

Good question! I don't think we can know for sure, but some EVP evidence suggests that this may be the case. When questioned the ghost box can reply that it is a spirit.

Also I've been on ghost hunts where professional investigators have researched the location beforehand ( usually over several days ) and have at least attempted to correspond the evidence with historical data.

Another thing is that many old locations such as castles seem to have greater paranormal activity, so one can at least guess that paranormal activity is linked to length of time. Another clue would be that many sightings are of apparitions wearing dated clothing, which would once again suggest an historical connection. Now I know you are sceptical of Mediums but this is another clue for believers ( and you did ask this question to believers). I would also add crisis apparitions and OBE to the evidence of a spiritual dimension to the paranormal.

Now lets be honest, when you go on ghost hunts there are a whole host of colourful characters ranging from spiritualists to secular ghosthunters ( like ghost adventures) to people who just want to experience something paranormal. So yes, a lot of folklore and urban legends also contributes to the idea that ghosts are deceased people.

1: find a location where there have been multiple sightings of the same or similar phenomenon

2: Investigate that phenomenon as best you can

3: gather any evidence you have

4: try to link up any clues to your idea.

5: Compare your ideas to others and see which seems to fit the picture best.

The documentary "Ghostbusters" instructs us how to tell the true ghosts from the false ghosts.

Of course, the best thing to do in case of suspected ghostly activity is to call Ghostbusters and request their assistance.

There's an episode of Dr. Who where ghosts start to appear all over the earth at the same time everyday. Everybody thought they were their dear departed ones but it was an alien invasion.

http://youtu.be/lNtTbrRVAec

I believe they are spirits

Not resurrection of dead people just spirits who takes the image of a dead person

They use their supposedly "evidence".