I could post pictures with circles and arrows, and lay it all out, but I prefer to make this a "teach a man to fish" reply. I will give the basics, and provide some links for further study, if anyone is so inclined. This information is also available in the Red Book.
As Sandon stated above, Trade Dollars had two obverse types. There were also two reverse types. (Skip Fazzari has done extensive research into these, and has identified several sub-types, which have differences in the eagle's feathers, the right end of the ribbon, and other places.) The type one obverse and reverse were used through 1876, so no coin dated 1877 should show any markers of either the type one obverse or reverse.
Here is a link to a site with pictures that show the most easily identifiable markers for the two obverse and reverse types. https://www.usacoinbook.com/coins/3169/dollars/trade/1876-P/type-1-obverse-type-2-reverse/
The NGC Variety Plus page shows which years has which types paired together, https://www.ngccoin.com/variety-plus/united-states/dollars/trade-dollars-1873-1885/819432/