Click on any tile in a battle to see what terrain type it is.

You should prioritize learning the spells for the more common terrain types first. Those would be Hell Ivy (used on Gassland, Thicket, Water Plant, Ivy), Carve Model (Gravel, Stone Floor, Stone Wall, Mud Wall, Tombstone), and Water Ball (Waterway, River, Lake, Sea, Waterfall). The absolute last one you should get is Lava Ball, since it only works on Lava and Machinery and there's only one dungeon in the game with Lava and it's completely optional.

Geomancy is nifty because each spell adds a status ailment to the attack. For example, Hell Ivy adds Stop. But you only reach the full potential by carefully placing your unit to take advantage of the terrain they are standing on.