How can you tell if an AR-15 is chrome lined

Freshly cleaned muzzle will have a bright circle (not always centered on the bore)where the Parkerizing stops.  Its usually a little larger than the crowning or muzzle chamfer.

On the barrel extension, the top portion of the locking lugs will be a dfferent color from the chrome.  If you shine a flashlight into the chamber, the color is much lighter and is a matte type of finish.

Chrome bores will polish with use.  New chrome bores will have a light colored matte finish.  New steel bores have a bluish tint.