Great interactive map at the site.
Gun Show Background Checks State Laws
Known as the "gun show loophole," most states do not require background checks for firearms purchased at gun shows from private individuals -- federal law only requires licensed dealers to conduct checks.
Under the Gun Control Act of 1968, federal law clearly defined private sellers as anyone who sold no more than four firearms per year. But the 1986 Firearm Owners Protection Act lifted that restriction and loosely defined private sellers as people who do not rely on gun sales as the principal way of obtaining their livelihood.
“Today, private parties sometimes sell large numbers of new and used firearms while claiming hobbyist status and exemption from the requirements imposed on licensed retailers,” according to Inside Gun Shows: What Goes on When Everybody Thinks Nobody’s Watching, a 2009 report from the Violence Prevention Research Program at University of California Davis.
Some states have opted to go further than federal law by requiring background checks at gun shows for any gun transaction, federal license or not. Five states, most recently Colorado and Connecticut, mandate universal background checks, an even more stringent standard that imposes background checks on almost all gun purchases, including over the Internet.
Even in states that do not require background checks of private vendors, the venue hosting the event may require it as a matter of policy. In other cases, private vendors may opt to have a third-party licensed dealer run a background check even though it may not be required by law.
View variations in policies from state-to-state in the following map:
http://www.governing.com/gov-data/safety-justice/gun-show-firearms-bankground-checks-state-laws-map.html
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