One of the most common InDesign graphic designer mistakes is when RGB image files are not converted to CMYK for print use. We can receive up to 200 individual ad files per month, depending on how many magazines are in production at any one time. We check each file's resolution, do a read-through of the copy for glaring errors, and make sure that the file meets all the specifications of the magazine, book, or website that it's going to be displayed in.
When an ad has been made in InDesign, there's a simple tool you can use to make sure all your imported images have been converted to CMYK. In the main menu at the top of the document, go to 'Windows', then scroll down to 'Links.' This will display a list of every imported file in your InDesign document, and will show its color properties. As you can see in the sample below, every image listed under the palette icon is CMYK. An image with RGB values would read as such, and need to be converted to CMYK (if it's being used for print).
This is a super simple, design 101 mistake - but we see it so often that it's worth writing about. It's also not the only one (don't even talk to me about images sent in Word docs, or headshots pulled from the web for print use).