The Diagnostic Spell is a spell that can be used in order to find out the cause of a supernatural illness.
When Sam calls on Rowena MacLeod to help understand what is wrong with Jack, she uses this spell in order to discover what was happening to him. The spell allowed her to learn that Jack's Nephilim nature as a half-human, half-angel is unable to sustain his body without his grace and thus he is dying.
Later, after the unsucessful recharging spell, Rowena performes the spell again only to confirm that Jack deterioration has not been fixed and all they can do is be there for him as he dies.
At first, the caster must incinerate magically a certain amount of unknown herbs in a bowl and then recite a Latin invocation over target's body, repeating the phrase:
Possim rei huius cognoscere causas. (roughly means "tell me the cause of this")The caster is then able to understand what is wrong with the person they are using the spell upon.