1. Levitation- This can be both quite disconcerting, especially if it is you or your plasma HD television that is levitating, or funny, as when it is your mother-in-law or the guy coming door to door to read the gas meter.
2. Bleeding walls- When noticed, you may want to ask the spirit kindly to stop, as it is heck on the carpet cleaning bills, especially on light colored carpet.
3. Clanking chains and going, "Whooooooo!"- Stereotypical and cliched (in fact, some ghosts actually find it undead-ist), but can be very noticeable, especially if they're trying to frighten away 'those darn kids'.
4. Kidnapping family members and allowing them to talk through the television- Rare, but it happens
5. Spirit Writing- Watch out; all spirits feel they are authors, and will be more than happy to force you to read their stupid novels about first teen love, or their really not-funny Fred Savage television pilot treatments, if you get them started with their spirit writing.
6. Possession- Can be fun if it is someone interesting, like George Reeves or Warren G. Harding, but really sucks if it's some nutjob like Hitler or Beelzebub.
7. Putting on a clown mask and jumping out of an empty room- I don't know if there is any of this on record, but you gotta admit, it'd be pretty wild.
1. Bad smells- "Well, honey, it was either the dog or spirit communication"
2. Cold spots- Did you pay your heat bill? If so, you may be in the presence of spirits.
3. Knocking and rapping- Especially if it is Tupac or the Notorious B.I.G.
4. Orbs- Floating balls of light, these are often signs of either spirit activity or binge drinking
5. Ringing bells- Usually occurs only if you have a bell available. If you are hearing bells without a bell available, it may be a brain tumor, or even an angel getting its wings.
6. Unseen presence- This is the feeling that there is someone or something near you, but there is nothing visible. If it is visible, then, well, it's a seen presence. And, if it's a seen presence, then you will probably have little difficulty understanding that a spirit is communicating with you.
7. Object movement. This is often very subtle, such as a ping-pong ball moving two inches over the course of three months, or tectonic plates moving across the globe over the course of a billion years.
This list is not, by far, complete, and may include any or all of the above in combination when the dead attempt to communicate with the living. Certain spirits may put a little more of themselves in their communication. For example, a friend of mine once had a particularly annoying spirit try to communicate with him by pitching softballs at his head. I've heard of other ghosts attempting to communicate with some ridiculous opening lines ("Hey, now, if I wasn't a wispy, spectral collection of ectoplasm, and I told you you had a great body, would you hold it against me, or at least against my decomposing corpse?") And, of course, now that I've given them the idea, you watch; you're going to see some copycat spirit go putting on a clown mask and jumping out at you.
You may want to sit down for that one.