You will probably find the aisle seats up the front i.e. closest to the front main doors are regarded as exit seats for the purposes you mention.
Don't know about the NGs but on the 300's I’ve dealt with, there are 10 over wing exit seats (the row of 4 in line with the doors and the row of 6 behind them). The placement of exit seats may not be immediately apparent just by looking at the doors so I wouldn't say they are 'clearly marked' (of course the exits themselves are).