If the effect is activated from the field, the effect is on the field. Examples are "Exiled Force" LV monsters, "D.D. Warrior Lady", Flip Effect monsters, TIV, Ignition Effects, ....
If the effect activtes in the Graveyard, then the effect is a Graveyard Effect. Examples are "Sinister Serpent" and "Witch of the Black Forest".
If the effect activates from your hand, then the effect came from your hand. Example is "Kuriboh"
Skill Drain only affects the monsters that activates and resolves in the field, as long as the monster is still face-up in the field when it resolves.
Skill Drain does not affect "Exiled Force" even though he atcivates on the field because when he resolves, he is now in the Graveyard. Same situation with LV monsters. This is also why if you manage to remove the monster out of the field before his effect resolves (with the help of "Ring of Destruction", "Compulsory Evac Device", "Trap Hole"), the effect will NOT be affected by "Skill Drain" anymore.