How to determine toxicity of a substance? I often read description of various compounds or elements as being poisonous. Now I know that through experience we have discovered the toxicity of various substances like lead, chlorine or phosgene etc. We know nitrogen isn't poisonous as its present in the atmosphere etc. 

What I wanted to ask is whether there is a perfectly analytical or experimental way through which chemists determine whether a newly discovered substance is or isn't poisonous? Or is it always found out only by exposure to humans (which I don't think is always possible)? Or perhaps whether such an information can't always be deduced?

I think animal testing is a possible (but immoral) method but again some substances could be harmful to them but harmless to us.
No, there is no way to determine toxicity to humans with a "magical method".

Most of our knowledge about toxicity is extrapolated from animal testings and from reported accidental exposures in human.

Also, Nazis carried out a large number of human testing on Jews, and this is not limited to toxicity.