Ballroom dancing has "tells" just like many other sports. A poker player may touch their ear when they are bluffing, etc. In my younger and more adventuress days, we used to informally race high performance motorcycles on a road called Skyline in the redwoods of northern California. Before you invited someone to "go down the road", you would always check out their motorcycle. It had very obvious "tells". If the wear pattern on rear tire was clear to the edge, and the foot pegs were worn down it was a pretty good indication that he/she would blow you off the road in a race.
I get phone calls all of the time asking: "Do you have live music at your dances"? This usually indicates that the person is just a casual dancer. Most ballroom dancers know that there are very few bands that can or will play strict tempo ballroom music and would rather have quality recorded ballroom music. Ballroom dancers are there to dance not sit and listen to music. I auditioned five or six "dance bands" and discovered that they didn't know what songs were what dances. The songs they played were 5 to 8 minutes long. This with breaks netted 8 songs an hour. We play about 20 songs an hour at our dances. They were also surprised to learn that they were supposed to play for the pleasure of the dancer not just play for their pleasure and the dancers could try to figure out what to dance.
A very obvious "tell" is whether a person is carrying or wearing ballroom dance shoes. If the person is wearing sneakers, boots, hard soled shoes, etc., they have pretty well defined their dancing ability. This type of foot wear severely limits the dancers' ability no matter how good he/she may be.
Ladies, if the leaders' right hand is down around your waist, he has the fingers of his left hand inter-twined with yours, or he keeps his head down looking at his feet, it is a very good chance he doesn't know much about leading.
When a man tells you that you are dong something wrong while you are dancing, it is because he thinks he knows how to lead and in reality he doesn't. 98 % of what a lady does wrong is usually the leaders fault. His "telling you" is the "tell". Don't take his remarks seriously. It is his problem not yours. I always advise the ladies, "Never apologize for not following. If he doesn't lead, you can't follow". Unfortunately, the lady can only dance as well as the man leads.
I talk to people all of the time that say "I know how to Waltz, Tango, Rumba etc." Any one that has any knowledge of ballroom dancing knows that dancing is an evolutionary process that never ends. You never "know" how to dance. It will always be a work in progress. A good dancer usually says when asked if they are good dancers "I am working on it".
"I like fast West Coast Swings" or "I like fast Boleros" are also "tells". WC Swings and Boleros are slow by design. The slower the dance the harder because it requires more connection, control and strength. When you dance fast you can just fall through the balance points, etc.
When a man says "I need more patterns because I am bored with what I know". This is a pretty good indication that he is a"pattern" dancer. A "pattern" dancer is one that just knows patterns and expects his partner to be able to dance the same patterns also. He usually can't dance with someone that hasn't taken the same classes. He has very limited leading skills and there is almost nothing going on between the steps. No matter how many patterns he knows, he will eventually get bored because he is not generating any feeling.
A good "tell" for the man is when women you don't know ask you to dance. This usually means that they perceive you as a good dancer (hopefully they aren't just desperate - Ha, Ha).
Ladies, just because you don't always get asked to dance doesn't mean that this is a "tell" about your dancing ability. I have seen cases where ladies aren't asked because they are too good or men think they are not available. Men can get very stupid that way. Take the initiative and ask the guys.
Men, a really good "tell" on how well you are dancing with a lady, is her smile. If the lady is smiling showing her teeth you are doing well. If she isn't smiling or there aren't any teeth showing you have some work to do.
Men, after you achieve the "teeth", the next level is to feel the ladies forearm when you escort her off the floor. If you feel goose bumps on her arm, you are starting to understand what ballroom dancing is all about.