This simple module provides a report showing where (in which entities) the PHP code format (from the core PHP filter module) is used. It also shows which blocks are using the php_code format.
By default this searches the revision table for each text field, as well as the block_custom table's format field.
Simply install, then view the report at Admin » Reports » PHP filter usage.
There are permissions for this module so you can extend viewing rights to specific roles.