You could maybe try going by the dates they were released?
That is actually brilliant. An obvious and logical approach that totally makes sense, no wonder I didn't think of it.
Unfortunately, there is no real standard as to how DLC is involved. The closest thing one can expect with handling DLC is that you have to go into the game itself for the option to be available. Typically the option for DLC will either be on the Game Start/Options menu or integrated within the game itself. As for seeing the difference in what's affected by DLC, the best advice I can give is to play without it the first time around and then pick it up afterwards if you feel like you're missing something (or find a guide on GameFAQs).
DLC usually is just an offset to the main story, a way to keep you in the game that you enjoyed so thorougly. That being said, if you like the game a lot and wanna keep playing, buy the dlc. It's just there to keep you playing after the main story is finished.
This was more or less my fear, and IMO the ultimate flaw in the DLC model for a gamer like me. Even some of my favorite titles rarely coax me into caring to play beyond the closure of the credits. In a perfect world, I feel like in game flags and notices that it as an appropriate time to engage DLC that ties into the narrative would be a great boon and selling point. In other words, if I got to "X" point and met "Y" character with notice that an accompanying quest could be purchased with focus on "Y" in the exclusive "Z" environment, the X Y Z combo would totally entice me into happily opening my wallet. I wonder why this bait tactic isn't a more common thing, if it is a thing at all.