How can I tell if a speck in my Nikon D3200 viewfinder is dust or a scratch?

If the mark only appears in the optical viewfinder and not in your photos, it is likely somewhere in the viewfinder path rather than on the sensor. From the description, a small round speck is more likely to be dust than a scratch.

A scratch is usually more fixed-looking and may appear as a line or irregular mark, while dust often looks like a small spot. The only way to confirm for sure is to inspect and clean the viewfinder components, but disassembling a DSLR can cause damage if you are not experienced.

If it is not affecting images, the safest option is often to leave it alone. If it is distracting, have the camera cleaned or inspected professionally. If you do attempt to open the viewfinder area yourself, be very careful, since improper disassembly can damage the camera.