Most upper receivers are not marked by the manufacture. Some upper end ones are, like Noveske, BCM, Daniel Defense and others, but not always. Some may have a small manufactures mark on them that you have to look for. BCM for example, will sometimes put a "BCM4" over the gas tube hole near the rail. Most uppers will have a forging mark. That tells you where it was forged. The most common forge marks you will see are the keyhole, which is Cerro Forge and the square, which is brass aluminum forging enterprises.