Mendelaines

I'm talking about a freshly assembled PC Board; assumbly all parts are put in correctly; Is it most often Chips, Diodes, etc; I haear a good tech can about figure the problem out by the symptoms. If this is correct, what are the symptoms of these chips, diodes, whatever? We once had a fresh-out-of school Tech who ruined a lot of our PC Boards by doing a process of elimination; I was just wondering about all this. Any Techs wanna respond to this?