It’s the truth either way. (Well, unless you look at all the new technologies and advances in science. Ok so, there are a few new things under the sun. Just not so much in advertising.)
There’s no way you can know everything, of course. But you should always check your idea before presenting.
- Google. Ask Google if she has seen that headline, tagline or campaign name somewhere else.
- Archives. Look at sites like Adsoftheworld.com, Creativity Online or Adland's Commericial Archive Search for the tagline, similar themes or images and seek out ads by your brand and other brands in the category.