→Construction Set vs. In-Game Formid
In the construction set (CS), you'll find the formid in the second column of the grid display of most windows. Note that this column is usually collapsed -- i.e. you'll need to widen the column to actually see the formid. For placed references, the easiest way to find the formid is to double click the reference in the cell view window -- the resulting dialog will list both the reference formid and the base object formid.
Note that the modindex may vary between CS and game. In both cases, the modindex represents the order of the source mod in the current context. E.g. the
boots from Knights of the Nine is 01000ecf in the construction set, but in game might be 0A000ecf, if KOTN is the 10th mod loaded after Oblivion.esm.
Note too that objects that specifically belong to the savegame and not to any mod are given the modindex FF. You'll actually find these quite commonly: all spawned npcs and creatures, plus all items initially acquired out of inventories and containers, plus all items created in game (spells, potions, enchanted armor and weapons) will all have modindex FF.