The NFL actually tracks "Average Separation". The issue, I think, is it is only tracking separation on throws to that player. As far as I know, there's no tracking of how open someone is or isn't when the QB doesn't look his way. That said, I think we can use it as a proxy--I'm willing to hear arguments against.
Right now, N'Keal Harry has an average Separation of 3.0 yards (Edelman and Byrd are at 2.7). That is higher than: McLaurin, Metcalf, Diggs, Evans, Tate, AJ Brown, J. Jones, Cooper, A. Robinson, Thielen, Chark, and Golladay to name a few.
The also track Cushion, which is how far the defender plays off of him at the snap. Harry is at 5.6. Cooper gets 7.1 yards, and Hopkins 6.9. However Metcalf is only 4.9 and D. Adams 4.4. Not sure what we can tease out of that.
Here's an interesting number though: Targeted Air Yards. Harry is only 6.7 yards. And yet he's got an average of 3.0 yards of separation. This is similar to Cooper Kupp, but mostly a lot of TEs. Most of the #1/#2 guys are over 10 yards.
Upshot--he's getting open, or at least he's more open when targeted than a lot of big name guys. He's just not very far downfield. He's also +1.1 in expected YAC, which is 24th in the league. Maybe we need to give him some additional routes or plays?