It's tough. I'm not even close to an expert, but here's my list.
1 - Base of the horns need to be bigger than the eye. Maybe even 1" bigger for really good mass.
2 - Prong should be above the ear. Mass should stay good up to the prong as well.
3 - The easiest part is the prong, it's either big or not. Pretty easy to tell.
4 - Total length - this is hard for me, especially when a couple inches is the difference between a really good buck and just a decent buck. I kind of do a thing where I look at how long it's horns are compared to how big it's head is. It needs to be the same at least. The tricky part is if there is a big curl or if it is fairly straight. I tend to see the fairly straight bucks and think they are monsters and see a buck with a lot of curl and think it's not that great.
If in doubt, pass on it, there are plenty of other bucks out there usually. One of the better pieces of advice I've heard is that you shouldn't shoot the first buck you see no matter what. There are a lot of 12" bucks out there that score 65 to 70 inches. I would guess you need to pass on 25 or 30 bucks before you see a good one in most areas.
That's my limited experience. Nathan