In an earlier post, I talked about using fakes to gauge the experience level of an unfamiliar opponent. To augment that, I want to talk briefly about what your opponent's stance reveals.
To discern anything from someone's stance, you have to have some experience (whether through observation or participation) in a variety of fighting styles. Most people without fight experience probably won't know the difference between a boxer's stance and a kick boxer's stance. So, step 1 is to broaden your fighting art familiarity. Know what fighters look like.
Step 2 just takes that knowledge deeper. Don't just correlate a stance with a fighting art. You need to know what types of techniques are characteristic of the art. Know how practitioners in that art tend to move. For example, even though Taekwondo and Karate are very similar arts, after spending 5 years in TKD and almost as much time studying Japanese arts, I can tell the difference between a TKD guy and a Karate guy just by the way they bounce around the ring. How is that helpful? Well, I know that both arts involve both hand and foot techniques (hence the name TKD...look it up), but my experience tells me that TKD people are MUCH more likely to throw kicks than hand techniques. In a fight, that would affect where I focus my attention, how I distance myself, and what I perceive as a telegraph of a technique.
There are A LOT of martial arts out there. TKD, Karate, Jujitsu, Muay Thai, Silat, Arnis,... The list goes on and on. The good news is you don't need to dedicate the rest of your life to familiarizing yourself with every art on the planet. From a statistical standpoint, ibwould recommend starting with the arts that are popular in your culture. In the USA, with the popularity of the UFC, I'd say to start with boxing, Muay Thai, and BJJ (yes, even grapplers have a stance. Many fights go to the ground, but almost every fight starts standing up). After that, perhaps learn about some of the more common traditional arts like TKD, Karate, and Judo. Recognizing a wrestler is a good idea too.
In another part of the world, the list of common arts might look quite different.
You can also take a more functional approach. Is the guy clenching his fists? Well, then he's probably planning on doing more punching than grabbing. Is he standing sideways ("bladed") to you rather than squared off? Well, then he's probably planning on kicking and keeping distance rather than punching and grappling. Is the guy standing up tall or squatting low? If he's up tall then he's probably going to strike. If he's down low then he might be going for a takedown.
All of these things can be discerned in a moment, and they reveal relevant information about your opponent. This being said, the opponent might be using his stance to deceive you, but that's another post!