It's hard to say that X in an attribute is NHL level or not... Some players with solid attributes underperform in the NHL while some players i wouldn't want in a junior team manage to do good things
However, in general, i tend to consider attributes of 17+ like star level, 15+ like solid NHL caliber, and 13+ like borderline NHL level. But it's not about one or two skills, nor the average of all the skills, but more likely the level in the skills that matter, related to the importance of those skills. For example, a goal-scorer with 13 in wristshot and 8 in flair can't be considered NHL level, while an enforcer with 10 in wrister, 5 in flair, but 18 in aggression and hitting, 14+ in the other defensive skills, a decent 11-12 in passing and stickhandling, combined with a decent 14+ in speed and acceleration is clearly someone that should play a role in a NHL team.
On the same way of thought, a defenseman with only 10-12 in his defensive attributes should seem to be AHL level (or ECHL). But if he's an offensive defenseman and has 17+ in passing, stickhandling, slapshot, and wristshot, combined with a 15+ in creativity, 13+ in flair, and 15+ in speed and acceleration, he should clearly be on your third pair and on your first PP unit! (but no PK time at all!

Never!)
Never underestimate the anticipation attribute. The game is really fast in the NHL, so if the player can't see how the play will devellop, he'll be useless most of the time.
High importance for the speed and acceleration skills too. A defensive defenseman with 18+ in his defensive attributes, 16+ in anticipation and decent passing and stickhandling skills should sound very attractive, but if he has 5 in acceleration and speed, he'll never be there to use his skills against a lightning fast forward like Ovechkin!
For the youngsters, they should be called up and make the big team once they have most of their technical skills at 12+. However, a very offensive forward should clearly make the big team if his offensive attributes are 14-15+, even if his defensive play is still weak (ie <10).
To sum up, take into account the skating level, anticipation level and the kind of player your guy is (playmaking forward, checking forward, stay-at-home defenseman, etc), and take those things into consideration when you're looking at the technical skills. It will give you a more precise idea about how skilled your player really is
Other than that, there's an easy way to know the quality of a player... Hire good scouts, scout the guy 3-4 times (or more

), and see what they say!
Hope that helps
