The best suggestion is just trust your nose; if it smells good to you, then it is good. You will know if it has gone bad, just because you'll ask yourself, "Is Chanel PE supposed to smell like rancid cheese?" Despite the fact that many companies say all of their juice from all batches smells the same, it just isn't the case (especially with smaller companies). I've had two different bottles of L'artisan Patchouli patch that smelled quite different, but were from two different batches. I tended to wear the one that smelled better to me, regardless of which one was technically "newer."
Also, Frederictoo, I don't think it is compulsive just to be curious about your own items. A date code isn't a serial number... it's a date code. Most of my experience with this comes from friends who work for some of the big Luxury goods makers (LV, Prada, etc.), where date codes are a big part of the game. With fragrances, this isn't as much the case, but it is still just fun to know something extra about a product on which you are spending lots of money. Does it really mean anything? No, not really, but quite frankly just the sort of discussion it GENERALLY provokes makes for a much more interesting thread than, "I think 4711 smells just like Chergui," "Please recommend me some Creed," or "Where do I buy authentic GIT?"