Easy, Google will tell you. As long as you have Google Search Console enabled for your website (which really is a must anyway) any serious issues will be pointed out to you there. If you want to do an immediate 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 if you head to GSC a full report which will include suggestions for improvement.