Sampling tests are not cheap though. Every test you do is whether interior or exterior is going to be around $300 or so. If you do not test on a pre-78 home, you must assume there is lead and do the job accordingly.
One thing I asked was this: If there is lead paint on an interior of a home that is not on the surface but under layers of non-lead paint, do I need to follow the lead safe practices on the job? It sounds like that is a gray area. The instructor said that we should be fine to do our regular procedures then, since the lead IS NOT BEING DISTURBED, so there is no exposure risk. Now, if the paint needs serious prepwork and the lead layers are exposed or become exposed, then you would need to follow the lead safe practices.
The test sticks are only so useful, since they are only good for a spot check of isolated areas, not good for a whole house testing. A whole house is tested with an XRF machine (doing a few hundred readings in the house) and will find lead in deeper layers as well as surface layers without having to sand to expose all layers (like the test sticks would need).