How can I found out if my daughter was claimed?

Well if you try to claim her and your return rejects.  Then you will have to mail your return and let the IRS determine who can claim her.  Did she file her own return and clsim herself?

if you didn't claim her, there is only one way.  claim her on your return and wait to see if you get a letter from the IRS asking you to prove your entitled to claim her as a dependent.  if you do get asuch a letter, unless you can prove you're right, be prepared for additional taxes, penalties and interest