One would make a reasoned estimate, relative to one's place and position.
Maghrib (sundown prayer) is easy: it is when the run has gone below the horizon. Isha (night prayer) is when it is now totally dark. Fajr (dawn prayer) is when light has appeared on the horizon. Zuhr (mid-day prayer) time doesn't very much from place to place. Asr is when shadows of things begin to extend themselves--and to be safe, pray it after half-way between Zuhr and Maghrib.