The World Wide Web Consortium has a checker for mobile-friendliness — the W3C mobileOK Checker. Type in a URL, copy a section of code, or upload a file. The checker will give you a score, a list of critical errors if any, and also a listing of changes you should consider. If you’ve used their other validators, this one should be very familiar and easy to use. A good first place to go to see how your site will fare on handhelds.