You can see the "borders" of a sector on the galactic map, and when selecting your hyperspace destination. If you want to find out the exact border locations, you will have to do it yourself, or buy those coordinates from someone ...
There can be hidden systems - systems that don't show on the galaxy map - but the actual sectors are clearly shown. A hidden system that's within a sector's borders will still show as part of that sector.
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."