Take time to think about what you are reading, looking for things that may be missing, phrases that are unusual, or anything else that seems "out of whack."
An 1867 act incorporating a railroad in Illinois granted townships the power to purchase stock in the railroad. When I took a closer look at the list of townships, approximately half of the ones through which the railroad passed were granted this power. If I had skipped over the list or mindlessly read it, I may not have noticed the omission.
Reading without thinking may cause you to overlook subtle clues--often the biggest clues of all.