You can usually tell if bark is alive or dead by the color. But you can also start pruning off the top growth, starting with smaller branches, and if you get to an area with living wood, the cortex which is just under the outer bark will have a greenish color. If the wood is dead, it will be all brown or grey. Since peach trees are generally grafted, you could possibly have a living root but totally dead top growth of the peach. There is no need to check below the bud union or graft site.