Seek out copy that is in small print. That is where many mistakes tend to cluster. Check the line breaks and page breaks. Transitions to the next line or to the next page are common problem areas, along with headers and footers. Look for symbols that come in pairs - parentheses, brackets, and quotation marks, for example.
Writers often leave out the second mark in a pair.
Hold the copy upside down to check the line spacing, word spacing, general format, and type quality. Use a straightedge or a sheet of heavy paper with a rectangular cutout to help you focus on specific areas of text. Use a horizontal cutout for ordinary text and a vertical cutout for tabular text. Read the copy out of sequence or backward. Consider reading paragraphs or pages out of order.