The usual way is to measure how long the person who answers the phone
speaks for.  A person might say "Hello?", "Hello, this is Alice",
"Thank you for calling XYZ.  How may I direct your call?"  Voicemail 
will usually be longer - "Hi, this is Bob.  I'm sorry I can't take
your call right now, so please leave me a message after the tone and
I'll get back to you as soon as I can."