1

Down and Dirty.  With your web site open in a browser window grab the lower right corner of the window and drag it all the way to the left to minimize the window.  If the content on your web site has rearranged itself to display in a more pleasing format appropriate for a mobile device chances are your site is mobile friendly.

2

Ask Google.  Google has a web site where you can check the mobile friendliness of your web site.  Take the Google Mobile-Friendly Test

3

Google Webmaster Tools.  If you’re already signed up to Google Webmaster Tools there is a mobile usability test that can provide a little more detail into issues you may have with your web site.