Is there a way to check for Tagline Copyright?

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I am participating in a few contest sites in which, I have to submit taglines. Now, a few ideas pop up in my head. But I'm not sure if they have been taken. Google, for once, is of no help here. The scariest part is, I have to check an Agreement saying that it is my original work. So, I can't really say. I need your help lotters. Please let me know if there's a Tagline Copyright checker or a way that I can check it - like you can easily check for the availability of a domain name. Thanks for any input.