Andy Wardley just wrote this reply on the TT mailing list:

Nope, there's nothing I can think of that will allow you to determine that from within the template. One work-around would be to INCLUDE a local BLOCK inside the template.

 [% INCLUDE my_local_block %]
 [% BLOCK my_local_block %]
    ....
 [% END %]
[download]

It's a bit clumsy but it does at least ensure that the template markup in the BLOCK will always be protected by an INCLUDE, even if the template is loaded by a PROCESS.