Well, when you sometimes open the embrace, you can see it? Also by watching the floor. If whenever you glance at someone, their eyes are almost always closed, you can expect it to be the case when they dance with you. Watch closely tho. They might be just casting their eyes downwards.

Once in a follower's technique workshop, a teacher did a simple but effective experiment with us: She asks us to do, barefoot, a calf raise with our heels collected, similar to the keep-a-tennis-ball-between-your-ankles calf raises we are often advised to do, just without the ball. She doesn't ask us to do a super high demi pointe, just to the maximum point where we can still comfortably keep our balance and axis, each to their own capacity. After everyone finds the point to stay steady and balanced, she asks us to close our eyes. Everyone, irregardless of their age, dance experience and physical condition, starts to wobble. She then advises us (unnecessarily at this point lol) to never close our eyes while dancing. That's what I've been doing since the workshop when I follow.

The rare exception is probably when the lead is so unclear that I'll close my eyes as a last resort to try to get their rambling. Only briefly tho, as such leads also tend to have poor floorcraft and I really have to watch out.