The one good thing about radioactive material is that very small quantities can be detected very easily using relatively simple equipment/techniques. A laboratory set up to do low-level detection can see incredibly small quantities of radioactive material in a sample.
If one really wants to know, there are labs in the US that will analyze a sample for you – probably would cost $100 or so for a single sample (perhaps more if they need to perform some chemistry on it first) e.g. GEL labs Environmental Consultants – Chemical Analysis and Radiochemical Analysis - Environmental Consulting Firm > The GEL Group, Inc.
