In most cases, you have a physical reaction. If you have an EMF meter, you may see the lights blink or a signal will sound. Sometimes, an object moves. If you have an IR camera or full-spectrum camera, do a sweep to record any apparitions.
Your body is probably the best tool, though. Your arm or neck hair will stand on end; you could feel cold or numb; you could even blackout. Unless you have a poltergeist haunting, you have nothing to fear.
On our investigations, Saleen seems to attract ghosts. She says her neck hair stands up. As for Jake, well, he’s about as sensitive as a brick, but he usually feels disoriented when something is near.