One way to tell if a high school diploma is fake is to check the document for spelling errors or continuity errors in font size or type. Another way is to check the paper on which the diploma is printed. Many diplomas are printed on heavy stock paper that has a glossy or waxy sheen to keep the document intact for many years.
Another option is to examine the document for any toner or ink bleeding or spotting. Diplomas are stamped and pressed, not printed on a commercial printer, making any blotches a telltale sign of forgery. Check the alignment of the text on the document to ensure it is perfectly centered and is consistent from line to line. This is an indication the document was not printed on a commercial printer, because commercial printers often change the page layout and margins as the paper is pulled through the machine.
Do research on the high school to determine whether it exists. If it does exist, call or visit the school to confirm the graduate's information or to compare an archived diploma with the diploma you are examining. Be sure to examine diplomas from the same year, as printing occasionally changes from one year to another.