Check colors in image

How about Window > Separations Preview?
https://helpx.adobe.com/illustrator/using/printing-color-separations.html

If you select that object, you can click the folder icon at the bottom of the Swatches panel to get all the used colors for that object.

You can use the Eyedropper to tool to sample colors in a placed pixel-based image.

Depending on the color mode, be it CMYK or RGB, all of the color channels will be listed even if none of the objects in the document have any color in a certain channel. That's the nature of that process color space. In order for Magenta to have a zero value that color channel has to be present in the CMYK color mode.

A print document that has only one or two colors could have those colors defined using spot color inks, where you pick a specific green. One color plate is used to print that pre-mixed green. In CMYK process printing it always has to be at least a combination of Yellow and Cyan, if not Black and maybe even some Magenta.