How do you know when ghosts are there?

Answer 1

Its a difficult question to answer because no one knows 100% if there is a way.

However...

The best ways are still experiencing them first hand. Seeing them, hearing them, or signs of them.

As for devices, depending on your school of thought and which particular theories you adhere to, you can use everything from a simple directional compass to sensitive ELF/EMF detectors, torsion field detectors, Ion Meters, and the like.

There are also some semblance of a theory that temperature can drop along with paranormal activity, so having a thermometer would be a good thing.

There isn't a real cinematic performance when they are there. There are no dense fogs, and creepy music.

Other than that its all going to be different for each ghost and each encounter. Finding some sort of trigger can often persuade the sentient spirits to work harder to show themselves, that's why some people argue with them.

Keep in mind, if ghosts are real, and if they are sentient, dealing with them would be no different than dealing with any other person on the planet, so use some shade of compassion.