M-16. If you have the auto sear, which can be detected from the exterior by the presence of the auto sear pin over the selector, or if the receiver has the pin hole and will accept an auto sear, even if it’s a stripped receiver with no parts in it whatsoever, it’s a machinegun. A selector that rotates 180 from "safe" as shown in the pic could be a hint that something's different. In a standard semi-auto set-up the selector is prevented from going to that position.


Auto parts on right, semi-auto parts on left. Note the “busy” selector—it has a few more features because it has a few more functions. Notice the hook at the top of the hammer, the long-tailed disconnector, and the open-backed trigger to make room for the disconnector’s long tail. My opinion, my advice, would be, “don’t have any of these in your semi-auto AR-15". Again I’m no expert on the regs and this is not genuine legal advice. Just saying, if it’s not papered as a machinegun, it would seem a good idea not to have any machinegun parts in it, although an M-16-style carrier alone, is specifically allowed.


Semi-auto trigger and disconnector left; auto parts right. Again note on the auto parts, the disconnector’s long tail and how the open back of the trigger allows it to fit in place. On the semi-only parts, the disconnector tail is shortened so a select-fire selector cannot act on it, and the wall at the trigger body’s rear prevents the installation of a long-tailed, select-fire disconnector.