Anti cheat engines are effectively spyware, except accepted spyware. How can you reliably ensure someone's machine is not running a cheat without having access

Anti cheat engines are effectively spyware, except accepted spyware. How can you reliably ensure someone's machine is not running a cheat without having access to what it's running? Anti cheat is not easy and just the perception of cheaters will nuke pvp games. But personally I want better anti cheat, the only question is at what cost?