Belts were pretty rare prior to the 1920's. Most pants were held up with suspenders. Folks like to spot firearms errors in movies, but if you see a cowboy, or anyone else wearing pants with belt loops it is not period correct.
or Cowboy hats. Admittedly, the Stetson was invented around 1865, so it did exist in the period, though not until well after the California Gold Rush days. Even during the Civil War era (and the Comstock Lode in the west) it was but a novelty. It certainly didn't crown many cowboys' heads until the 20th century.
Near as I can tell, if there was any one hat that was more common in the old west, it would have been the derby/bowler. This wasn't unique to the west, but characteristic of the whole US. Gentlemen (the rich or powerful) wore top hats. For the working class, flat caps were popular in the west as well as all over, but probably more so in the cities and where people worked in factories. The bowler was the most popular brimmed hat. If a man got by well enough, his hat looked like a bowler. If he wore rags, his bowler's brim was probably drooping and looked tattered or he just wore a soft hat without any particular style -- a frumpy piece of felt; see Walt Whitman.
In many ways Stetson's first "Boss of the Plains" hat was like a bowler with a flat wide brim, mixing in some of the mexican sombrero style. The common sombreros back then did not have the comically wide brims seen on some today, but still would have been wide enough to be unfashionable with anglos. The modern "cowboy" hat really came into with the Cattlemen of Texas -- not what I consider "the West." The brim is turned up on the sides, supposedly to keep the lasso from knocking it off. In Alta California, the Vaquero (or "Buckaroo" if you couldn't pronounce Vaquero) wore a flat-brimmed hat.
Another hat that was phenomonally popular in the late 19th century and early 20th century, the first period in which the "cowboy" hat could have enjoyed any popularity, was the straw boater. I've seen a photograph of miners on the Comstock Lode in the 1880's where one was wearing a boater. The others were wearing bowlers in various conditions. The boater really came into popularity in the early 20th century and in the summer. During this time, the Bowler had given way to the Homburg as far as fur hats were concerned. Although the Homburg had its own history, it was in the practical sense, a Bowler with a center dent. By the 1920's, the Fedora was eclipsing the Homburg and continued in popularity until men mostly stopped wearing any kind of semi-formal hat at all. Now for nearly sixty years we've had no hat popular at all but the ball cap, usually with some kind of legible inscription, emblem, or logo.
