If you have a chunk of metal or a rock that contains metal and the metal contains >4% nickel (Ni), then it is probably a meteorite. If the metal contains >0.02% chromium (Cr) or manganese (Mn), then it is not a meteorite, however.
If the metal contains <4% nickel, then the metal chunk or rock is not a meteorite.
If you have a rock that contains between 1.0 and 1.8% nickel (whole-rock analysis), whether or not it appears to contain metal, then the rock might be a meteorite.
If you have a rock that does NOT contain metal and has a low concentration of nickel (<1% = <10000 ppm), it could still be a rare type of meteorite, an achondrite. (About 5% of stony meteorites are achondrites. The probability is exceedingly small, however, because nearly all (guesstimate: >99.999%) Earth rocks have the same properties - no iron-nickel metal and low concentrations of nickel (<0.3%)
http://meteorites.wustl.edu/id/metal4.htm