Which Lead on a wall wart is positive/negative?

There is no set standard. Usually the wall wart will have a diagram printed on it which will tell you the polarity of the connector - whether the tip is positive or negative. If you don't have that then you will need to have a volt meter to test the connector and/or the leads.

As for your second question, it might be possible to connect two wall warts in parallel. However the voltages would have to match exactly. With voltages that do not match exactly you run the risk of feeding back the output of one unit (the one with the higher voltage) into the other which could possibly burn one or the other out. I think you would be safe if you diode isolated both so neither one would feed back into the other. Another problem you might face is the load would most likely not be carried evenly so one unit would be working harder than the other and that might burn it out too.

Use a bulb and diode with diode arranged to allow current to bulb. Apply wire of diode to a wire and the other wall wart wire to bulb side if bulb lights that is the positive lead .If not that is the negative Simples

It varies.The wire coloring, IME, is more dependent of the polarity cord to the plug rather than the cord to the supply, in that the striped wire usually connects to the tip, regardless if it is a positive tip, negative tip, or AC supply..

That said, it is most typical that the tip is positive, but you ought to check to be sure.