What should steamed milk look and taste like? I think the better question is what should it sound like? I thought I was pretty good at steaming milk when I started training with one of the world’s best latte artists. I could steam it spending about the right amount of time stretching it and heating it, hitting the transition at about the right time (and more particularly temperature). My milk tasted pretty good and I was proud that I knew about things like the stretching phase even existing, not to mention knowing when to enter and exit it and how to tell based on temperature without even using a thermometer.
All of that was important stuff to know, but the very first day I trained and the very first pitcher I steamed my trainer left the room as I started and from the back of the coffee bar could tell me that it was not going well and that my milk would stand no chance of being micro foam. That, of course, meant that it could not turn into latte art, but it also meant that the taste could be dramatically improved.
The sound that I now hear in most coffee shops that warns me I should walk right out is something like an airplane taking off. Ideally the sound should be far more subtle. “Tst tst tst tst …” followed by a peaceful but quiet swirling of the milk.
You can almost always tell by the sound if the milk will be quite good or not. It makes for a fun party trick. It also is a great key when trying to learn to steam milk. Listen to a video with a real pro steaming and then see if you can reproduce the sound. If you are not working on a machine as powerful as they are you will be making the sound for longer, but ideally it should be similar none the less.
If your machine is massively underpowered you may not be able to do it at all, though. For example I usually just pull shots on my lever and then turn it off. If I do this and out of habit on a day when I choose to steam milk and turn it off before steaming I always know it as I’m steaming because the sound is not right and the milk never pans out well (it is a reasonable, but not spectacular steamer if the user isn't quite so careless).