Mostly writing this so I have somewhere to point people who ask me how to do this…

1. Browse to C:\inetpub\wwwroot\wss\VirtualDirectories\
2. Locate any web applications you are interested in (Central Admin can be identified by the port number)
3. Edit the web.config files with the following:

a. Find the “CallStack” attribute and set it to “true”
b. Find the “customErrors” node and set it to “Off”

4. Browse to C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\TEMPLATE\LAYOUTS  (or C:\Program Files\Common Files\microsoft shared\Web Server Extensions\14\TEMPLATE\LAYOUTS if you’re in SP2010) and modify the web.config there with the same changes, this allows errors on pages in the _layouts directory.

Source: MSDN – Working with Web.config Files