A classic book: Distinguishing Bolts from Screws, U.S. Customs and Border Protection, March 2006. [AIR 16:2] This 21-page book presents both technical specifications and common sense advice, such as:
U.S. Customs and Border Protection uses fastener industry standards to distinguish bolts from screws. When a fastener is described in a fastener industry dimensional standard as either a screw or a bolt, we follow that standard. When we have no dimensional standard, we go to “Specification for Identification of Bolts and Screws,” ANSI-ASME B18.2.1 1981 (the “specification”)….
If it doesn’t meet the primary criteria (and of course, if it doesn’t conform to a fastener industry standard for a bolt), then it probably is a screw.