The source of lead can be hard to determine as there is so much lead in the environment.
Also, I can assure you that if you are in a house with horse hair plaster, it would have plenty of lead paint unless TONS and TONS of remediation was done. Almost nobody would do such remediation as it would probably be cheaper to demolish the building and rebuild.
Also for what it is worth, I believe the current lead scare is a bit overdone..... My understanding is that on average the current generation of children has less than 1/4 the lead levels that their parents had as children.......
Plus, both of my children tested high as infants. I lost too much sleep over this when my kids were toddlers.
Today my daughter is an 8th grade honor student and has already taken the SATs with over 1300 total score. My son is in 5th grade reading at high school levels with simalar scholastic achievements.....Go Figure!