When Siri first debuted it would automatically beep, so I could immediately tell if the phone did not recognize recognize "Hey Siri" (just "Siri" didn't work). A couple of iOS updates later this went away, which means I can't tell without actually picking up the phone and looking at it whether the command was accepted.
Even more annoyingly, sometimes there is a beep! ¯\_(ツ)_/¯
