How do i know if the problem is electrical or the battery/alternator?

i have a 95 olds achieva. i replaced the battery a year ago and the alternator was tested and was ok. I have driven a tremendous amount of highway miles since last summer .The last last two months..strange flickering of lights have occurred.when the key is not even in the ignition...mostly when i first get in the car..or have disengaged the key from the ignition and am about to get out....plus the internal "dinging" noise as if the lights were left on when you were exiting the car. Now very recently the car is not turning over..but yet the interior lights work...and they blink seemingly out of control without me doing anything. Almost as if the hazard lights were on. It was able to be jumped but I just want to make sure this not a loose wire or connection before i go take into a mechanic. Additionally..any idea what an alternator for a car this old would go for..I am in the northeast