How can I determine why my lights are flickering?
Flickering is caused by a bad connection somewhere.  It could be the bulb or socket that is bad (you did try a new bulb, right?), or it could be a bad connection anywhere along the line: at the fixture,  switch, any electrical box along the way or a fault in a cable (staple, screw, etc.).  It could be the hot or neutral that is bad. A sign of the neutral being open is other lights getting brighter or dimmer. While the ground not being well connected would not cause the flickering, it might be a sign of other bad connections along the way.

You will need to visually and physically inspect all of the connections with the power turned off.  Wiggle wires, look for ones that pull out of connections.  Look for nicks or otherwise degrading wires.  Check the wire insulation too. Look for heavily oxidizes wires or connections.

You need to get yourself a non-contact voltage tester! You shouldn't be testing live circuits with a screw driver, that's a going to get you killed.


(source: homedepot.com)  

Call an electrician if you are unfamiliar with electrical work.