Describe extraterrestrial life?

If we do find extraterrestrial life wouldn't it be completely different from the lifeforms we are used to on Earth? For all we know extraterrestrial lifeforms probably breath toxic gas or something instead of oxygen or drink liquid mercury instead of water. The genome of extraterrestrial life forms is probably completely different from the organisms on Earth or perhaps they might not even use double stranded DNA or single stranded RNA as a blueprint. Maybe they have a hexagonal-like structure for their genetic blueprints or they might not even use genes. So when we do find extraterrestrial matter, how can we clify it as a life form or just a bunch of molecules bunched up together to make some sort of complex organised chemical elements?