That is a really good question.
I normally look to see who it is and make sure I have a weapon close at hand...especially...if I am not expecting someone.
We have had more than a few instances of people checking to see if someone is home and then jumping the fence to break in the back door.
I live on a corner and it is pretty easy for someone to jump the fence.
If it looks like someone conducting a survey, I look for an id badge or I don't answer.