The metal in iron meteorites consists of two alloys that together are commonly called iron-nickel (FeNi) metal. The metal in iron meteorites (also, that in stony irons and pallasites) strongly attracts a cheap ceramic magnet. So (this is simple), if you have found a piece of metal that is not strongly attracted to a magnet, then it is not a meteorite.
Humans have been making and losing metal things for thousands of years. Much of that stuff, almost certainly anything manufactured before 1900, is mainly iron. Iron metal, e.g., wrought iron and cast iron, also strongly attracts a cheap magnet. (Some stainless steels do not attract magnets.)
If it looks like metal and attracts a magnet, then you have to have it analyzed for iron, nickel, manganese, and chromium to determine whether it is man-made or an iron meteorite. However, it is often easy to tell that it is manmade just from the shape. Iron meteorites are rounded but not spherical. There are no straight lines, right angles, or flat parts. I have never seen an iron meteorite that was long and thin. Also, unless it is very rusty, you cannot break an iron meteorite and it will not have broken sides. Here are some photos people have sent to me of objects that look like iron meteorites to me.
All of the photos below were sent to me by people who thought the objects might be meteorites. I suspect that many of these things were found with metal detectors. All this stuff is man-made.