Full Idea
Perfect knowledge of the end consists in not only apprehending the thing which is the end but also knowing it under the aspect of the end and the relation of the means to that end.
Gist of Idea
We must know the end, know that it is the end, and know how to attain it
Source
Thomas Aquinas (Summa Theologicae [1265], II-I.Q132), quoted by Philippa Foot - Natural Goodness 4
Book Reference
Foot,Philippa: 'Natural Goodness' [OUP 2003], p.54
A Reaction
We don't talk much now about 'perfect' knowledge of something, but I suppose this is the necessary and sufficient conditions. If you complete the checklist, your knowledge should be perfect (if the list is right).