The device is often a small bar of metal or tab which protrudes from the firearm when a chambered round pushes it outward. The bar is often colored red on top to make it more visually distinct The tab projecting from the firearm also allows the state of the weapon to be determined by touch rather than requiring a brass check.
These have become very common, and some areas (eg California and Massachusetts) have laws which require them on all new-production handguns. Many law enforcement agencies also stipulate them as a required feature for service sidearms.
Some criticism has been directed at the concept by firearms enthusiasts, who feel that an LCI encourages poor gun safety, in particular ignoring the typical "the devil loads the guns" mentality to treat a firearm as always being loaded. Prominent loaded-chamber indicators also cause potential snagging issues.