You should take the following steps to ensure a design is based on 100% original work and doesn’t incorporate any clipart, stock imagery or, in the case of logotype contests, licensed fonts:

• First, ask the designer. Designers must declare any non-original work used, or risk account suspension.
• Listen to the other designers participating in your contest—they’re the experts, and can often see a plagiarized design from miles off.
• Try a reverse image search engine such as TinEye. This will look for similar images on the web.
• Double-check the designer doesn’t need to supply you with any fonts—text should be rasterized or saved as an outline in the final deliverable.