The IRS will not tell you. If you say in My Info that you cannot be claimed as someone else's dependent and your own e-file is rejected due to your SSN being used already, you will know someone else claimed you. Then you need to print, sign and mail your own return instead of e-filing and let the IRS sort it out. And they will.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.