how can I get the custom field ID?

Inside mail templates it's possible to use <custom_ID> tags where ID obviously is the ID (an integer) of the custom field, eg.:  <custom_12>.

Unfortunately, I couldn't find the place in which the ID of a custom field is explicitated; it roughly corresponds to the Display Order, but since that one can be changed, the two may differ.

Where is the authoritative info?

thanks in advance