How to check polarity?

Look for an Elfix Polarity tester.

Yes, using the first bar for a dedicated line keeps your stereo AC out of phase with the rest of the house and is a good idea. However... you might want to spring for a qualified electrician. If you accidently touch those innocent looking bars you could be dead before you hit the ground.

1. There is no positive bar.
2. Both bars are identical; no "first" or "second" bar.
3. Correct with respect to what? It's 120 volts to ground.

It does not matter from where you take your power, either bar is exactly the same: 120 volts AC to ground, not to neutral. The only place where polarity applies is at the receptacle itself where the voltage is with respect to neutral. The power has to come from the black wire instead of the neutral. If it comes from the neutral (reversed polarity), then your equipment might not perform to spec. That's why you check the polarity of the receptacle (not the panel), and the reason why some plugs have a wider blade on the "hot" side. Other devices, such as lamps, do not care which way the power comes from so it doesn't matter if the polarity is reveresed.

As a rule, always grab power from both bars of the panel in an equal manner. That is, do not pile up your circuits on only one leg because you will draw power unequally which can result in lower power quality, hotter wires, and your 240 volt motors (air conditioners) will burn out quicker.

Also, if you do not know about the basics of electrical power systems, it would be a good idea, before you go poking around, to have an electrician or an ambulance standing by.