It is mid-July, 2015. Google has just started sending notices to webmasters of sites that implement hreflang incorrectly. Distilled has launched an open-source tool for SEOs to test their sites’ Hreflang implementation. This was much-needed because well-known international SEO consultant Aleyda Solis’s tool to generate hreflang tags, while useful, wasn’t something that SEOs can use to validate hreflang tags on hundreds of pages on their sites.
Since I launched the Spanish version of Diffen in December 2014, I have been using a tool I wrote to validate hreflang tags on the site. And it has saved my bacon on several occasions where it pointed out all sorts of errors I made in tagging the Spanish and English versions of the site.
Over the next several weeks, I plan to polish the tool and convert it into a web service so that webmasters/SEOs can use it to test many pages at once. I also want to compile other helpful resources for hreflang like this guide from Google.