A Fresh light is a green light you know for sure is going to stay green. The only way to be 100% certain is to see the pedestrian “Walk” signal advising the pedestrians that they may start walking across the crosswalk.
A Stale green light is one that you think might change to yellow soon. Clues are: if the light has been green for a long time; if you see the pedestrian “don’t walk” signal; and if there are any vehicles waiting on the other street (on the traffic sensor) and/or pedestrians who have pressed the pedestrian buttons.
Traffic lights don’t just change randomly; they change due to demand from vehicles on the sensor or pedestrians.