You can read the share condition in the bottom left of the in-game scoreboard. (The map name line there contains the share condition, the map size* (in km), and the map name.)
IE: Full Share - 20 x 20 - Seton's Clutch
* Some maps set the playable area to only be a portion of the overall map size. So, the playable map area can be smaller than the overall map size in some cases. For example, a 15 km x 15 km map will say 20 x 20.
You can scroll down the options list and see what is listed for 'Share Conditions'. If you click to enable the 'Hide default options' checkbox, then it can remove a lot of clutter from the list. If you do that, the share condition will be displayed if it is full share (or anything else besides share until death, which is the default). So, if you hide default options and the share condition is not listed, then it is set to share until death.