In the photo they appear to have a raised-lettering backmark, which on dug-in-the-US brass (or copepr) 1-piece flatbuttons means manufacture between about 1790 into the late-1830s. On buttons from that time-period, the word "gilt" meant gold plating, and the word "plated" meant silver plating. Your buttons are made of the legendary British Sheffield Silverplate, a thin layer of sterling silver bonded onto a layer of copper.
You asked for their value. Dollar-value of ones with no silver remaining is about $1... at least, that's what you can buy "dug" (excavated) ones for at relic-shows here in Virginia. Dug flatbuttons with "a lot" of silver still showing and looking pretty can bring $5 to $10.
You asked about how to know if you've found a Tombac buttons. Tombac is an alloy of about 85% copper and 15% zinc. (That's less zinc than is typically found in yellow brass.) Consequently, actual Tombac is an "orange-ish" or "golden" color. Some modern-era Canadian 5-cent coins were made of Tombac. See the photo below.)
What most diggers call a Tombac button is actually made of "White Tombac"... which has a dull silvery looking color. It is the usual copper-&-zinc Tombac alloy except that it also contains a small amount of the metallic element Arsenic, which has the effect of turning the alloy to a silvery-ish color. Perhaps the presence of a trace of Arsenic is why (White) Tombac buttons tend to come out of the ground looking nearly as clean and shiny as the day the were lost.
the second one is a little larger than the first but says the same thing
