Look for the issuing school district. Even a home-schooled student must take his exams through a particular school district, because home schools themselves are not accredited. Find the school district and make sure it is a real, by looking in a phone book, or online, or by simply asking around.
Contact the school district that issued the diploma. Often this is the only way to tell whether one has been faked, because there are so many schools and each has its own type of diploma. Also, there are many businesses that create fake diplomas which look realistic, so you should verify with the district that a student actually graduated.
Check the diploma for an accreditation agency. Every school that issues diplomas is accredited, which means that an accreditation agency has certified that the school teaches what it is supposed to teach. Even home-schooled students must pass tests set forth by an accredited agency in order to get a diploma for graduation in their state. Contact the accredited agency to double-check that the diploma is real.