20110408pg upgrade fix: How do I know if I am being affected before errors occur

Hello!

In connection with the recently discovered bug with pg_upgrade:
http://wiki.postgresql.org/wiki/20110408pg_upgrade_fix

If I an not sure whether pg_upgrade or pg_migrate has been applied on
certain databases in the past:
 - How do I know if I am being affected by the bug _before_ errors
occur?
 - Can applying the proposed fix break anything if the db wasn't
affected to begin with?

Information as to that is missing in the mail to pgsql-
announce@postgresql.org as well as on the wiki-page.

Thank You! Regards
Erwin