There are some visual observations one can profit from, although dancing with someone is decisive for me. Some are universal and style-invariant, some are not:
0. The embrace: some embraces simply look painful for me, independent of a particular style. But I'm a leader, so I choose the embrace, more or less. The followers might want to choose something that looks comfortable. Most of them do, here.
1. The facial expression and the whole body of the follower can show relaxation or a certain degree of stress. I admit, it does take a lot of experience to develop the ability to perceive this. This has nothing to do with the potential "bliss" or whatever, because sometimes the music can be so intense that the follower cannot express the bliss she is experiencing. The facial expression can bias the overall observation.
2. Assuming a relaxed follower, the leader will then also seem relaxed and the quality of the lead will express itself through a good musical interpretation, i.e. you will be able to recognize his particular style after a few songs. I've seen many good leaders look... confined... with bad followers. Not beginners, just bad followers. I know a follower who always seems tense... She's not a bad dancer, but some things are impossible to do with her.
<2.5. It boils down to the fact that you can only have a good idea of how good a couple looks like, as someone already pointed out... so it could be that 2. leads to 1. in some cases.>
3. Once relaxed, the moves. Oh yes, is is easier to see the bad dancers who move a lot, but crapily. But also, a wider spectrum of musically used steps/figures/... executed in a visually appealing way shows that the couple has the technique but also feels relaxed and confident enough to use them. (Of course, keeping in mind all the time that those steps are not the point here, but the relaxation and musicality that lead to them.)
4. Last, the musicality. And it is last for me, not because it is not important, but because it is the most important and also most difficult thing to evaluate, being the most subjective of all. And here's my question: Can you let go of your idea of musicality while looking at others? As many couples you can make, that many different musicalities you can get. I keep trying to shake off my own interpretation (the idea of how to dance to a song) while watching others and find it possible, but very hard to do. Listening to comments to someone's dancing, it seems not everyone is aware of this... I hope you'll realize what I'm talking about: "he SO didn't interpret that violin", "what music is he dancing to" or similar. There is a continuum of interpretations out there, for each and every song!
Regardless of the answer to that, it's always nice to see a follower express the layer of music I intentionally left out for her to fill in. I LOVE that.
5. Oh, forgot one thing: the floorcraft. Easy to spot, but mostly attributed to a good leader. However, those "unlead boleos" are a bigger red flag than an occational bump, IMO.
OK, this turned out too long. Sorry if I repeated some things already mentioned, I just wanted to condense my thoughts after looking at the thread for a few days...
