I used to work in a lab, and for many years I've had to go in for regular lab test, several times a year, and each time it's always spelled out whether the test is fasting or not. If they haven't told me, I call the lab or the doctor's office and ask.

It really matters with some tests, and labs are pretty strict about it. They usually don't want you taking in anything with any food value (including diet soda) after about midnight.

It is dreary to drag myself to a lab in the morning without anything to eat but I bring food along with me and eat it right after the lab test. A small can of V8 juice and a stick of string cheese and a breakfast bar, usually--something I can carry along with me without spilling. Not my favorite way of starting the day but for the sake of an accurate lab test result, I go along with it.