Margins of victory in NC games would be a good one... and especially when compared back to conference games against supposed lesser opponents. NC participants are a product of the competition they faced during the year.
It is somewhat difficult to compare bowl games since SEC teams frequently play "up". IOW's, a 5th seed SEC team plays another conference's #2 team.
A legit match up to compare by this year was UGA vs Neb. The game wasn't as competitive as the score... in spite of Richt not being a great game day coach. Those types of games where repeated often become a basis for comparing the conferences with less subjectivity involved.
The matchups of teams in comparable positions within their conferences is another. Bama hammered Michigan when you were probably the 3rd or so best team in the Big 10. UF dominated tOSU a few years ago in a NC game. When the top tier of one conference pretty consistently beats the top tier of other conferences... that's an objective measure. You can look at mid vs mid, lower vs lower, mid vs lower too.
Players going to the NFL provides a good indicator taken over time.
i understand what you're saying but i don't think it's quite that straightforward and simple. i don't think anything done in regards to a NC game relates because you're talking about one team and not the conference as a whole and in regards to participants being a product of the competition they faced, IMO that says less about them in that they didn't necessarily play the "best" teams week in and week out to lose either 0 games or 1 game, and win them that handily.
and while it's true the SEC does play up, the big gaps are against conf USA, big east, and the ACC while the others are generally playing "up" one spot. then you have to consider every SEC bowl game this past year was played in an SEC state. not saying this automatically gave them home field advantage but it's a variable and has to be considered. that being said, overall since the BCS started, the records don't show the SEC as being the toughest/strongest/best (or whatever you want to call it) conference, as i've said before, having the best team doesn't always equal being the best as a whole.
Ga/Neb i agree would be two teams that i were to sit back and look at would probably say are fairly matched so i'll give you that but again, one sample piece does not determine the whole.
you can look back at games here and there that can suggest one thing or another but wouldn't you be better suited to look at overall records as a conference rather than this big name team vs that big name team? as far as mid v mid, lower v lower, etc, there might be some clear cut cases but i think that still comes down to opinion of who you ask, and really anything can happen in any game on any day to give you an outcome you completely didn't see coming. NFL players, maybe. do you look at ones that just made it there, do they have to have a certain level of success to count, what about the ones that flame out after one year or the ones who played a season(s) in the SEC as well as season(s) in another conference, how do you count that guy?
one other thing, i'm not looking at this totally from the conf v conf view, i'm also considering toughest to play in week in and week out and the competition you face every week. would you have a more balanced overall record top to bottom or would you have a clear three tier definition with 0-2 loss teams at the top and 0-2 win teams at the bottom?