How can you tell a gas is a halogen?

They are right next to the Nobel gasses on .

The halogens are five non-metallic found in group 17 of the periodic table. The term "halogen" means "salt-former" and containing halogens are called "salts". All halogens have 7 electrons in their outer shells, giving them an oxidation number of -1. This periodic table shows the position of halogen gasses which are: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At).