How do I know if the artwork I have is high resolution or not?

You may be able to interrogate the file yourself, depending on the file type and the software you have. In Photoshop, open the image in question, then click Image>Image Size. In the dialogue box that appears, the lower portion will tell you the size of the image at its current resolution setting. Make sure that Resample Image is UNCHECKED, then change the resolution to 300 pixels/inch (ppi). The resulting height and width under Document Size will tell you how large the image can be printed at 300ppi. If you are unsure, please let us check the file for you.