Within using Sketch (version 3.4.4), I can see the CMYK color conversions for a specific layer by using a built-in feature.
I went to the toolbar and selected “View/ Show Colors”. There, you’ll find a color wheel and a few other options. I used the eye-dropper tool (bottom) to grab the color of some layer in my project. After you’ve grabbed the color, click the “Color Sliders” from the dropdown menu, then choose “CMYK Sliders”, or hit Command + 3 for short.
Even though you still have to grab those initial percentages when you enter Illustrator/ Adobe program of choice, I guess it’s nice to know in advance what these percentage values will be beforehand.