Know your state's regulations to determine whether they are being followed by the homeschooling parent. Some states have very strict homeschooling laws, but other states have no laws at all. Be sure you are reporting homeschooling practices that are actually against the law. See the link under the Resources section to locate the laws for your own state.
Understand that poor academic performance in one or two subjects is not necessarily an indication of bad homeschooling. Children in any type of school setting may have trouble with a subject from time to time. If possible, keep an eye on the situation. Consistently poor performance in numerous areas might be the result of inadequate homeschooling.
Document the evidence. Notate anything you see that is an indication of poor homeschooling such as lack of instruction or poor academic skills in most areas. If you have any supporting evidence such as standardized test scores or other measurements of the child's progress, be sure to hold onto them so that you may turn them in to the authorities.
Contact your local school board. Depending upon the state you are in, this may be the department that governs homeschooling.
Contact your local department of children's services. If the school board does not govern homeschooling and cannot tell you who does, contacting Department of Children's Services is the next logical step. In some states, this department oversees homeschooling practices. If not, they should be able to tell you who does.
Contact the state's department of education. If no one on a local level can take your report, your state's education department can. In many cases, this agency will have a sub-department dedicated solely to regulating and overseeing home school practices.
Understand that an investigation takes time. If the authorities feel that there is compelling evidence of inadequate homeschooling, they will likely send out a case worker to investigate. From there, the homeschooling parent may be counseled regarding proper homeschooling practices or the child may be required to enroll in school. Regardless of the outcome, it can take weeks or even months to reach a resolution.