You need to enter admin and run a query (how to run a query: http://orcanos.com/forum/viewtopic.php?f=36&t=955)
Then run the following queries:
Custom fields that are in use:
select o.Obj_name as 'Project name', c.Field_name ,s.description as 'object_type'
from ITEM_DATA i , objects o, SYS_REF_VALUES s, CUSTOM_FIELDS c
and i.Obj_type =s.code
and s.table_code ='REF_OBJ_TYPE'
Custom fields that are not in use:
from CUSTOM_FIELDS c
where ID not in (select custom_field_id from ITEM_DATA)