1- How do you know which alter you are?
That's easy. How do you know which alter YOU are? It's no different for multis than for singletons. We exist with our own sense of consciousness. I can only speak for myself, but I believe it's true of others in my system as well. When we're in control we just ARE. We're not an "alter". The others in my system are aware that they are "alters", are aware of my life (mostly), and know that they're just temporary. But that doesn't change how they perceive their own existence. To them, they just "are", and all else (including other alters in the system and including me) are "others".
2- How do you know you are not *the* alter itself?
Maybe I am.

I think that the host ("main person") is just another alter. If I am an alter I don't mind. My goal is to become healthy, complete, and unified (either through integration or co-consciousness). If my function is temporary and I am an alter, I am fine with that.
3- Are alter's lifelong?
Unless something happens to them, yes. Therapy can integrate them, or they can disappear/go into hibernation and new alters can come out.
4- What if you spent more time as your alter(years) than you did as 'yourself'? Would that mean to your original personality? That it is now the obsolete one?
I don't know that there IS an "original" personality in my system, or how I would identify it. I'm fragmented because of intense and ongoing childhood trauma. If there is an "original" personality, it's locked up inside to keep it safe, and I doubt that it would be able to come out and be functional. My "original" personality would be something like 4 or 5 years old. How would that personality function in the life of a nearly 30 year old woman?
The alter that spends the most time in control is technically the "host". So if I don't spend as much time in control over the years as one of my alters, then it's a good indicator that I'm an alter, and not my "alter".
5- Would you change anything(Be a whole Identity, no alters)?
Are you happy the way you are?
My alters have some skillsets and talents that I would love to possess. I am also a little bit paranoid that their ability to do certain things with as much finesse as they are able to achieve may be related to the fact that I am fragmented. If they were weighed down by the realities of everyday existence would they still possess that capacity for brilliance? Would integration result in something more bland than the sum of my parts? So I'm mixed. The idea of integration is very appealing and somewhat disturbing at the same time.
My main goal is really to eliminate some harmful behaviors and cope with the trauma that I suffered so that the others inside aren't still suffering and aren't still acting out on it.
6- Do you believe that there are people that fake this disorder, or are not truely experiencing it?
It's a hard disorder to fake. It requires the ability to hold a tremendous amount of information in your head. From mannerisms to memories to mood sets. It's not something that's easy. It's like a one-man Shakespearean improv that lasts over years. If someone CAN fake this successfully I'd love to meet them.
That said, I'm sure that there have been people that have tried to fake it for SOME reason. People can be incredibly damaged.
7- Do you you know anyone that claims to be DID but is not in your opinion?
Some of my alters insist that I'm faking DID. I'm sure you see the humor in that statement.
8- Do you have control over when you switch?
Sometimes yes, sometimes no.
9- What does switching alters feel like?
It varies. Usually colors go all wrong and my eyes go out of focus then I fall asleep. Or sometimes I watch and feel out of control. Or sometimes I watch and feel "I should stop this" but have no power or real desire to stop it. Sort of like being at the dentist on Nitrous Oxide as they drill. You sometimes feel the pain and you know that they're drilling holes in your mouth, but somehow you have no ability or desire to do anything about it.
10- How do you communicate with them? Is it a voice, or a thought?
It's thoughts that I argue with, debate with, that try to convince me of things, that say things I disagree with, that chatter constantly.
11- Are your alters aware of you? Are they aware of one another(if multiple)?
Yes and no.
12- Do you have just one alter?
I wish.
13- Why do you think you have alters? What are their purpose?
That's a difficult one. Primarily to offload different parts of traumatizing experiences to different parts of me and keep the "whole" broken up so that I don't ever have to deal with the actual emotion or the reality of things that have happened to me.
14- Does anyone outside of your family know of your condition?
Only three people know.
15- Could you say all of your alters are similar to yourself in some way? Like a piece of you that is taken to an extreme and given it's own identity? Or are they entirely different people?
Some are very similar, some are polar opposites.