All on-chain transactions come from a known EOA that your contract can verify.
// Checks that the caller is the known EOA. Ensures that your callback is only called when your midpoint returns a response.
require(tx.origin == 0xC0FFEE4a3A2D488B138d090b8112875B90b5e6D9, "Invalid EOA");
Live callback EOAs:
Look for 0xC0FFEE when checking an address.