Check the websites (see References) of the major high school accrediting bodies to determine whether the school is accredited. It's often normal for a school to be accredited by only a single organization, so it is likely that you will have to check more than one website to find which agency certifies accreditation for a particular school.
Research the major college accrediting bodies as outlined in Step 1, keeping in mind that, unlike high schools, colleges can be accredited by more than one accrediting body. This is often the case for schools that offer highly specialized programs in addition to general education degrees.
Contact the administration office for the high school or college in question. A member of the staff should be able to provide you with a copy of any accreditation certificates it has received from a legitimate accrediting body.