Sometimes you may have a run away reindex process that aborted due to a MySQL connection error of some sorts. It may be useful to use the following MySQL commands to aid in debugging.
Magento Enterprise labels the reindex lock via the constant REINDEX_FULL_LOCK in app/code/core/Enterprise/Index/Model/Observer.php
SELECT IS_FREE_LOCK('mydatabase_name.reindex_full')
Returns 1 is specified lock is free and can be acquired, 0 if it’s in use, NULL if an error occurs.