Bolt gauges can be a help for some. Your local hardware store should sell something like the first one at this link.
http://www.atlanticfasteners.com/tech_help/gauges.html I grew up with a huge advantage in this department. Dad would fill his pockets out of the scrap bin at work at an aircraft company. Every few weeks I would sit on the floor with a bucket of hardware with fasteners of all styles and sizes, sorting them out. At the age of 5 I could tell the details of a vast number of bolt sizes and styles, including jo-bolts, hi-loks and huck bolts.
Now, if I could only find a way to use that talent to make money.
