Compare to what level of spell the Supernatural Ability is equivalent to. Use that to determine if it's too powerful, especially if it compares to an extremely strong spell of that level (wish) or a stupidly weak one (meteor swarm). Then you have to take into account permanent benefits (WBL boosting, perma-minions, or perma-buffs), or if it's a fairly throwaway ability, even if it's somewhat useful (magic missile).