We delayed at least 2 years to convert our main websites from Desktop visiting support only to responsive UI, which means a Mobile-Friendly website.
How we know whether our website is mobile-friendly or not mobile-friendly ? The easiest way is using existing online tool. We met multiple tools in past couple of years, but along with the time changing, the mobile-friendly criteria was keeping on changing also.
The most possible reliable tool now might be the one from Google: Google Mobile-Friendly Test online tool.
Just input your website URL, you will see the result like below:
If the website is mobile-friendly, you will see the result like this : Awesome! This page is mobile-friendly. (so you see it actually only test the page it caught the 1st one)
If the website is not for a mobile device, you will see the below result: Not mobile-friendly: