How to characterize gaseous ethylene that forms as byproduct?
If you want something simple and don't mind less than 100% certainty then you could bubble the gas through Bromine Water - in the presence of an alkene the bromine water would be decolourised from brown when reacting with the ethene.  Of course this does just indicate the presence of any alkene so isn't exactly specific! There is more about this on this website.