How do you find the source of lice?

Person to Person. Whether you're talking about head, body, or pubic lice, rhey are spread from person to person. It is difficult to find the exact person they might have come from, since it can be awhile from the time of exposure to when you notice the lice. What is important is treating it, and depending on the type of lice you're talking about that possible contacts are treated as well.
Why? Why is it important to find the source of lice? The important thing is to remove all nits from the head after treatment. Wash all bedding and clothing in hot water. Repeat nit combing in 7-10days. Where you got the lice, is not really going to help you. Even if you avoid that person or ask that they are treated, you can be exposed elsewhere. Never share combs, brushes, hair pieces, hats, hoods...