You may fear that someone claiming you as a dependent will eliminate your personal exemption — you can claim yourself as a dependent if nobody else does — but a stranger cannot pick a name out of a hat and claim you as a dependent. If someone illegally does so, he has to deal with the consequences, not you. However, you have to prove that this stranger cannot claim you as a dependent.
TL;DR (Too Long; Didn't Read)
If you have been illegally claimed as a dependent on a tax return, you should print out a copy of your tax return and mail it to the IRS alongside a cover letter explaining that you are independent taxpayer.
What To Do
If you feel someone mistakenly claimed you as a dependent on his tax return, do not electronically file the return. Instead, print out and mail your return. Include a cover letter explaining the dispute and why you are an independent taxpayer. Usually, proof of independence is all the Internal Revenue Service needs to process your return and possibly penalize the taxpayer who illegally claimed you as a dependent. The IRS won't reveal the name of who claimed you as a dependent due to privacy issues.
Claiming Independence Opportunities
The IRS allows taxpayers to claim someone as a dependent if that person meets certain guidelines. An adult dependent must earn less than $4,050 a year, pay for at least half of her own expenses, not file a joint tax return, be a citizen of the United States, Canada or Mexico and be a nonrelative who lives with the taxpayer for the entire year. A relative doesn't have to live with you the entire year as long as that relative meets the IRS's test, including being a child, stepchild, foster child, sibling, parent, stepparent or child of one of your siblings. If these requirements do not apply to you, you likely qualify as an independent.
Other Important Considerations
You may want to ask family and friends if they claimed you as a dependent. Sometimes this can work to your favor. For example, maybe your parents claimed you as a dependent so they can keep you on their insurance plan. If you can find out who claimed you as a dependent, the individual may need only to file an amended return using Form 1040X. On a 1040X, report the change in deductions in Column B and explain the reason for this.
In Case of an Audit
If the IRS cannot determine whether you count as a dependent, it conducts an audit on your return and that of the person who claimed you. You may want to hire a tax attorney to represent you through the audit process, especially if you think an audit may reveal errors on your return, such as unreported income, that increase your tax liability.