After the first few days of use if they answer then it will be a machine, because caller ID will tip everyone else off to avoid your company and your products. Do you believe that anyone in the world likes to get robo-calls?
That said, I knew someone who worked on a system to do this (not on Asterisk) and did voice recognition on variations of "Hello" followed by a pause to indicate that it was a person and not a machine. A machine greeting will usually be "Hello" followed by a message like "you have reached ...". To be very simple, you just wait 1 or 2 seconds and see if there is a pause for a reply or speech.