Most modules expose variables with the naming convention Err... that represent the errors that they can return. The errors can be compared with those values (eg. err == os.ErrPermission). Some modules also define error types that provide additional error data, such as net.OpError; a type-cast can be used to check if the error is of that type, and then access the data within. As a fallback, all errors implement the Error interface which allows you to display them as a string if you can't handle them based upon their type.

It's not unlike exception handling by exception type, although it's definitely more of a manual process because Go doesn't give you specialized syntax for it.