Easy, Google will tell you. As long as you have Google Search Console enabled for your website (which is a must anyway) any severe issues will be pointed out to you there. Suppose you want to do a quick test, however, to look for the smaller things that will improve your site’s SERP positioning, head to Google’s own Mobile-Friendly Test Tool. Input your URL, and you will be given a basic score and then head to GSC for a full report which will include suggestions for improvement.