The list worth carrying around: weight dropping week on week, a tail visibly thinning, sunken eyes, shed that will not come off, nothing out the other end for a long stretch, any swelling or mouth trouble. One of those and the season is not your explanation.
Juveniles are a different animal. They are growing and should not be fasting for a fortnight, so if anybody reads this about a young gecko, ignore the reassuring half of the thread.