Hi Kathryn,
namaste,
There are any number of ways you can know if one has attached.
But let's say, as someone else has on the other thread. Nearly everyone has 1 or more attached.
Some, but not everything to look for:
cravings that are unusual for you. Needing alcohol, tobacco, etc., when those are not what you usually want.
mood swings
hearing voices
depression
Health issues you've never had before.
Hearing yourself say it... as in: "it's time we got ready to go to work", and no one else is around.
Understand that having any of these does not mean that you do have an attachment, but are a reason to take a look.
If you are in a place where you can see your own aura, then look. Or ask someone you trust to do it.
I usually pick them up, because I have sent others to the light, and my aura has a marker. They are pretty easy in that case to talk to them, request the presence of relatives that love them and are waiting, and help them leave. Most are just confused, didn't know they had died and now don't know how to cross over.
There are exceptions to that rule, however, and some people are vulnerable enough to pick up "nests".
I have had my guides change my marker so that I am not just covered over with them all the time...it's office hours, so to speak.
I don't know if we can prevent it in the first place. Firekeeper may have some ideas about that. Basically it is clear, clear, clear.... Salt baths, smudging, etc. Just be vigilant.
But as some have already said, they do leech your energy, so you really don't want them hanging around.
Hope this helps,
Priestess