How do you know when a dusty antique from your attic is worth something?

Go easy with that stuff you may just find many treasures, WW ll stuff can be worth something, the object you described is a Viewer, it's probably 19th century or so,. When you handle stuff Do not polish or repair anything, Look for any Signatures, Hallmarks, or any identifying marks on objects. Sort them out and put into a category, like the WW stuff, China, Silver, Paintings, Especially papers and photographs, if they match any item that makes them more of value. example: if you have an Aviators flight jacket, medals, and military papers to along with it, it makes a package with verification etc. Make sure you look up items here on the NET and if you feel it's worth something, contact someone who knows antiques and appraisals. I hope you find a fortune. Good Luck