The classic test for an alkene is to shake your sample with orange bromine water, and if the molecule contains a C=C double bond the bromine water is decolourised.
A chemical reaction has clearly occurred, but could you draw out the mechanism?
CH2CHCH3 + Br2(aq) ⇾ CH2BrCHOHCH3 + Br–(aq) + H3O+(aq)
If we want a dibromo-product we need to change the conditions of the reaction, with bromine dissolved in an organic solvent at room temperature.