How find that a color is in CMYK color space or not (other than adobe softwares)?
Other free software such as Krita and GIMP are capable of showing when RGB colours are out of the CMYK gamut for a particular CMYK ICC colour profile. You don't have to use Adobe software for this.
Although GIMP doesn't support CMYK mode itself (but Krita does), you can still enable colour management in the preferences, and download a CMYK ICC colour profile to use from the International Colour Consortium website.
Then you can go into Edit > Preferences and enable colour management, and use the CMYK ICC profile you downloaded for the Soft-Proofing profile. Then you can go into View > Colour Management, switch on "Proof Colours", and switch on "Mark out of Gamut Colours".
Here's an example I made. I have painted some blue lines in GIMP using 00ffff which is an out of gamut colour for CMYK printing (shown on the left), and the view on the right shows colour proofing enabled, with the out of gamut warning enabled. GIMP's default is to display out of gamut areas with a purple overlay.