Rule of thumb: flat ribbon or strip wire is generally 1870s–1890s. Plate and odd "patent" barbs are mostly 1870s–1890s. Glidden-style 2-point barbs on round wire have been made since 1874 and are still made today, so a date is always a range, never certain. Heavy, bright galvanizing and 15.5-ga stiff wire usually means modern (1970s+).
Disclaimer: This is an approximate field reference only. Many patterns were made for decades, copied by other mills, or reused from older fence. Patent dates show when a design began, not when a given fence was built. Don't rely on wire alone as evidence of a line's age.
Sources: NPS Homestead National Monument barbed wire brochure, The Henry Ford barbed wire collection records, US patent records, and collector references (Antique Barbed Wire Society, Devil's Rope Museum). Each pattern lists its source.