I have no trouble telling the diff....
in fact, I make strong practical use of the differences for specific needs.
For instance, if there is noise in my environment that pretty much kills the ability to hear the music correctly, it's my closed-back Beyerdynamics for me.
If the environment is quiet, I go with my open cans every time, as they present a far more open stage, a greater appreciation of space. More like being at a live performance.
And sometimes, when I want a REALLY rich, DEEP experience, I will run my speakers while I am wearing the open-backed phones, put myself in an optimal spot between the speakers and become ABSORBED into the music.
I listen to quite a lot of classical music in this way.
Here's a really good way for you to test whether you can hear the difference or not;
Put on a pair of open-backed phones.
First notice, without music playing, that you can hear sounds through the mesh from outside.
Next, put on some nice music, maybe not head banger, just something like jazz or classical.
Bring your hands toward the external mesh- notice that you start to hear a dampening of the stage, a dampening which increases as you bring your hands closer and closer.
Cover the mesh with your hands, then take your hands away.
Repeat until it becomes clear to you that you are, indeed, hearing a distinct difference between your cans when open, and when closed.
(This procedure effectively eliminates driver distance as a factor in your appreciation of the sound difference, and prevents one from being able to blame differences in headphone brands and builds for sound changes, if one were switching between 2 headphones to test it out.)
You can play with closed cans in this way by lifting the edges away from your face and noting the increased sense of space when the edges are a few mm away from contact.
You CAN tell the diff.