In obsolete|lang=en terms the difference between detect and nauscopy

is that detect is (obsolete) detected while nauscopy is (obsolete) the supposed ability to detect incoming ships, before they come into visual range, by studying certain atmospheric effects.

As a verb detect

is to discover or find by careful search, examination, or probing.

As an adjective detect

is (obsolete) detected.

As a noun nauscopy is

(obsolete) the supposed ability to detect incoming ships, before they come into visual range, by studying certain atmospheric effects.