It is not easy to learn to read another species' body language. Heck, it's not even easy to learn another person's body language or facial expression. How many times have you thought that a new person you've met was expressing an emotion that they weren't aware what they were conveying. I mean, we've all heard of Resting Bitch Face. Still, though it's not perfect, we use each others body language and facial expression to convey and receive all kinds of important information. There are a lot of great resources online to get you started on reading dog body language, but truly spending time with you pup will help you notice patterns best.
So many people misread their dog's reactivity as aggression. Just because a dog is making a lot of excited noises (even growls to a degree) does not mean that an attack is imminent, although it also doesn't mean that your dog is just "really happy" to see their friends. But how we respond to that excitement as handlers and trainers should be consistent. We should be asking for an alternative, redirecting that energy into focusing on what we ask via commands, and waiting until the dog is not so excited to move onto any next step- especially entering a dog park. Dogs have evolved to communicate with us as part of their species survival strategy, but it feels like with more anthropomorphizing, we have lost the ability to really pick up on their cues.
Each dog that I have met has shown me slightly different combinations to communicate anxiety, feeling unsure, excitement, interest, etc. It takes time with a dog to watch what happens when I introduce stimulus that is unexpected, and find any patterns that present throughout different distractions. Once I have a decent read on a dog, I can better see if their behavior starts to change. For example, right now I'm working with a gorgeous brindle pup, whose name is Stiles. Stiles is a dog who is coping with some fairly deep anxiety, which has recently started developing into near fear-aggressive reactivity. Loud, sharp barking, growling, and teeth snapping when he is stressed has given his family really strong concerns. So I'm working with him to build up his confidence, and defer to his handlers when faced with something new or potentially scary, instead of escalating his behavior. Stiles has a really sweet smile- but it becomes really "stretched" at the corners of his mouth when he is nervous. A quick glance and someone may think that he was either flashing his teeth, or just super happy. But what's actually happening is he is clearly telling me, before he starts barking, that something is upsetting him. He also tenses his back and pushes his ears high and forward. Now that I know this, as a trainer I can take him just to that line, and then reduce the stimulus and offer counter conditioning so that we can increase his tolerance positively, and at a safe speed. But it is only because I observed Stiles and have practiced learning to read dog language that I will be able to help him with problematic behaviors. Rio doesn't display this same anxiety sign, rather she hunches low, or bounces up and down like Tigger when she is nervous. Badger pants and licks his lips, Swanson whines and hides.
Your dog absolutely has cues for when they are bored, when they are hungry, sick, stressed, cuddly, relaxed etc. The best thing to do when learning to read your dog is to think about whether their needs have been met, and correspond it to the behavior you are observing. So for an easy example, every night at about five pm, our dogs know that dinner is coming. They may get up, walk over to the food and water dish, stare at me, stand near the door for potty break (always before dinner), or some combination there of. They know that food is coming, and they are reminding me of the time. Swanson, if he hasn't had enough activity for the day, will come and stare directly at me, wagging at a very slow speed, and stomp his front two feet until I get the leash or redirect his behavior elsewhere by asking for a lay down or sending him to his crate. I try to project what I think the dog is communicating by remembering that dogs love patterns- so if an action corresponds to a daily routine- they may be linked. It's not enough to just notice those patterns though- we then have to figure out what to do if the escalation from the warning sign is a problematic behavior (like leash reactivity). Often a dog may also develop a behavior that's serving as an inappropriate coping skill for boredom or stress. Dogs that obsessively lick, pace, circle, etc- are all usually missing enough mental and physical stimulation throughout their day, and find that this behavior either gives them something to do or causes a human to interact with them (good or bad). Occasionally behavior patterns can cue a family into a health concern as well. Our lab mix guards food much more actively when he is sore in his hip.
One of the best things one can do as a handler or owner of a pup is to read and study the most current information about body language, and then see what you can look at in your own pups and use it as a jumping off point for correcting behaviors. Aside from practice, one of the most important aspects of dog training is prevention! If I can head off a behavior before it hits peak severity- then I can reward the dog sooner for an alternative. A good jumping off point to learn more about canine body language is to check out this webinar from the ASPCA and this post from Barkpost!