Assessment is not what social tango at the milongas should be about.
MsHedgehog wrote a terrific post, Called "Diagnostics" about leaders who perform a sort of diagnostic as they dance, trying (usually just 1 or 2) different movements and gauging their follower's responses. In this way the leader is able to determine her skill level, and comfort level, and meet her where she is in her dance. I also tried to explain something similar in a post called "Leading from the Bottom Up".
Leaders, if you can do this subtly, by which I mean undetectably, this is a wonderful tool. It's a process that shows you care about the dance you're giving your partner. That gentle, subtle technique is not always the case, I'm sad to say. I frequently "feel" a leader running through a checklist like a sort of pop quiz I haven't studied for. Here's how it feels to me, and to the woman who wrote the email I referenced:
Can she do this? No.
How about this? Nope.
*deep, exasperated sigh*
lather . rinse . repeat
I'm not sure this is common knowledge amongst leaders, but when leaders let out a disappointed sigh or make some sound of disapproval, I hear it. When you frown after I've missed a lead, or let your frustration show in your breathing, I feel it. And when I feel those things, I don't always mean to, but I often shut down my connection to you. (Which of course makes my following even rockier - see the spiral here?) I do this because when you apply a checklist and conspicuously grade my dancing in a social setting, you're hurting me.
I understand that some or even most of that frustration may not be directed at me, but it feels directed at me. And it felt directed at the author of that email. This process of assessing the follower, is not the same as meeting her where she is in her dancing with empathy, or "Dialing In" to her , or understanding what she's comfortable with. This dance process becomes about the leader's dance, how the leader looks and how the leader feels.
For me, social tango when it's at its best, leader and follower are working together to express and play within the music. So it's not about what leaders are able to make their followers do. It's about what we can express together. When we're dancing socially, for fun (remember that fun bit?), there are no mistakes (unless someone gets hurt or is disrespectful - that's different than what I'm trying to address). It's not a test. It's our time to play in the music together. Some things work, other things won't. Social tango is an art, and aiming for some sort of perfection, is the opposite of the expression of that art.
It's different in classes, lessons and practicas. In my lesson with Grisha and other teachers, a diagnostic checklist is in essence what I am paying for. It's what I was there to get. I expect to be tested more, get feedback, address weak points. Those are the places for that kind of thing.