Re: Question about OP AMP
If you just want to know what is the positive and negative input, you can do what dumbfrog suggested.
If you want to understand, you can do this by imagining that the voltage at one of the inputs rises and the voltage at the other input falls, and see what happens to the output voltage (following the path of the currents from the input transistors until you reach the output node). Something like "If the voltage at this input rises, than the current on this transistor will rise, which also increases the current on this other transistor (...) and then the output voltage will fall".