When trying to clean out stale custom fields, or better organize them, it would be very useful to be able to see which Page Layout(s) the field is already available on or what formula depends upon it. Why not add to the Custom Field Definition Detail screen a label called Page Layout(s) Available to indicate where this custom field is at least available. Another Label could indictate if this field is part of a calculated field formula. If it is not available anywhere, you know you are safe to delete it. If it is available, at least, you can see whether it is crucial to another fields (part of a formula).