Ghosts or ghost-like presences manifest themeselves as
sights,
sounds,
smells, or even
feelings. They are usually found at locations of high positive or negative emotions, such as anger, love, envy, or joy.
Ghosts frequently appear as ectoplasmic mists and brightly lit spheres (called orbs) in photos. Actual human shaped sightings of ghosts are the rarest of all ghostly visitations. http://www.ghostweb.com has a large library of ghost pictures, including "vorteces", mists, and orbs.
Ghostly sounds are relatively common in hauntings. Sounds of footsteps, loud banging, creaks, and voices with no apparent source can indicate ghostly presence. These, like ghostly smells, may occur more often than many realize, because of the ease with which one can discount them to natural phenomenon, or even just voices in one's head.
Ghostly smells, and I don't mean the smell your dog makes after he steals a burrito off the table, may also be more common than most people realize. Frequently reported smells include cigarette smoke, flowers, and perfume. My wife has had several instances in our house of an unfamiliar, strong perfume smell. It's not anything we use in the house, and it comes and goes without a discernible pattern.
Feelings may be the most common hauntings of all. Feeling that someone is watching you, or that you're not alone, or even that something evil or good is present are all ways that ghosts haunt. Again, these can be easily dismissed as 'just feelings.'