Distinguish between an oxidising flame and a carburising flame in oxy- acetylene welding?

Basically, you need to look at the flame itself, if the tip of the nozzle is bluish/white, and long , then the flame is a carburising or reducing flame, it contains a high ratio of acetylene compared to oxygen, and will produce a cooler sooty flame. this can be used to make metals more malleable when working them, but the carbon is not helpful, and cannot be used for welding operations

An oxidising flame is more pupleish/white and is far hotter, with more oxygen than acetylene, this flame is used in cutting, but will harden steel, making it brittle.

in between the two is the neutral flame where welding operations are carried out.

Images can be seen at: http://www.twi.co.uk/content/jk3.html (not my site)