How do you tell if your American silver eagle is a pf70 or not?

You can't unless you pay the $20+ to send your coin to NGC/PCGs even under magnification the ordinary person cannot tell the difference between a pf69 and pf70 coin. Of course, it is easy to tell if your coin is uncirculated or proof, a proof silver eagle will have very shiny, mirror like fields that are easy to see yourself in. An uncirculated silver eagle will be shiny, but won't be mirror like. Also, the raised part of the coin will look frosted on a proof coin but not on an uncirculated coin.