A DDR will show you what layouts, relationships, scripts and value lists a field is used in and is a very helpful option to determine if you need a field anymore. I would also examine actual data in the field to see if that is stored in another field before deleting it.
Take a look at BaseElements | Goya Pty Ltd
It performs an analysis of the entire DDR and will show you if any field is referenced (used) , unreferenced (not used), or potentially indirectly referenced (not sure if used or not. you need to manually look for references)
beezwax > products > inspectorpro 5 is here! is another option but i cant recommend it because i've never used it.