Which module facilitates looking up database schemas of custom content type?
I think the Schema Module can handle that...


  
  Schema documentation: hyperlinked display of the schema's embedded documentation explaining what each table and field is for.
  


I confess I haven't tried it but the module page mentions 


  the INFORMATION_SCHEMA database that the Schema module uses to inspect the database.


so I imagine it has the functionality you're looking for, plus inspection of the live db if you need it.

Or if you don't mind wading through a large krumo array you can always use drupal_get_schema()

dpm(drupal_get_schema());