The spell description says "An unseen servant is an invisible, mindless, shapeless force that performs simple tasks at your command."
I assume "at your command" means some sort of verbal statement, rather than a mental intention.
Can I order a servant to follow the commands of another party member?
Can someone disguised (or polymorphed, or cloned, etc) as me command the servant?
Can a magic mouth give commands to a servant? A programmed illusion?