Keepers often times get a 6 because they had nothing to do. If your defense plays very well, and you aren't playing a strong team, he might see no tough shots the whole match.
I think an "average" keeper will score about 6.5, but it's hard to judge because it's dependent on other factors.
My keeper right now averages 6.8, I've had him for 5 seasons and he's always been really good. I tried to replace him with an 18yr old super fast trainer, who didn't do well at all, so I went back to this guy, and he's still been good.

So keepers, really the only way to judge is the eye test. When you play vs good teams does he stop their corners and counter attacks? Does he let in cheap long range goals?
No other good way to evaluate your keeper imo.