The Guaranteed Wardrobe Unlock is an item from the Black Lion Chest that you can use to randomly unlock a missing item from your wardrobe. It can be any skin except exotic and legendary gear, and you can also unlock miniatures, colors, clothing sets, and kills.
In order for this potentially valuable item not to meet a simple skin for a few silver or karma, Reddit user immortius has developed its own tool . This tool will show you all the missing skins on your account, and also show you whether you can create them with Karma, Gold, Gems, or Craft, or earn them through achievements or reward paths, for example.
All you have to do is create an API key in your Guild Wars 2 Account Manager on the official website and enter it into the tool. Content can be read out directly from the account via the API key (eg which equipment you own or what is in the bank), but it is not possible to access your account.
The tool is visually still in need of improvement and immortius has already announced that this will be a next step. Although GW2Efficiency offers an equipment overview, we find this tool much more helpful when it comes to unlocking skins. The only downside is the language support, since the website is only available in English.