How do I know if my photo resolution is good enough?

Our app will ensure photos are good for print before allowing you to add it to cart. A message will warn you if we detect photos which will not print beautifully. In general, we aim for a 300 DPI print quality. This means, if you have a single photo on a large 30cm page, you will require a photo of about 3500 pixels width (about 12 Megapixels, which is most iPhones)