This Crock-Pot brand stoneware / black glazed ceramic liner was positive for lead at 42 parts per million (ppm). [Tested in July of 2014, exact year of manufacture unknown.].
This is a level that is considered safe by all standards (United States and European.) It has also likely been leach-tested. That said I would personally not be comfortable cooking food for hours upon hours in a vessel that is positive for any amount of lead, especially when there are lead-free options out there.
If you have to use a crock pot or slow cooker I recommend this one (link), because it has a stainless steel liner (even though the cooking element is not 100% lead-free, at least the vessel that contains the food is.)