No there is no way of knowing this.
No you can't.
You can normally tell by the number of followers that they have. A lot of celebrities are starting to have "verified" accounts too which means Twitter have verified that they are the real person.
With the whoami command.
No this is not possible.
Sometimes, it depends how well your twitter account is protected and how many people search you. ;)
If something has changed that's how you would know.
Tweet them.
No, there is no way of knowing who views your profile.
There's no way to know who has viewed you on Twitter. However, you can set-up your page as "private" to protect your tweets.
The friends of the person's account could tell them when the last time they signed on was...? Hope that helps.
You can't unless they are tweeting and their tweets show up on your timeline .
You could tell them to stop. Or you could report it to Twitter