Most people say their dog knows sit, however the definition of "knows" tends to vary quite a bit. Depending on who you ask some dogs know sit when they hold a treat up, some know sit when pressure is applied to the leash, and some know sit when it is repeated multiple times.
My definition of a dog "knowing" a behavior is that the dog can perform the behavior the first time it is cued, no matter where they are or what is going on. Your dog should be able to perform said behavior without bribing, without the threat of a punishment or correction, and you better to be willing to bet $100 on it.
If your dog does not "know" a behavior that is okay, this just means your dog is still learning. However there are ways of teaching a behavior to ensure that your dog "knows" it and it is reliable in all situations.
The first thing we need to start with is capturing, this is when you allow your dog to offer a behavior naturally without any cueing, luring or prompting. Let's use sit as an example, you would start by simply waiting for your dog to sit, once they do click/treat immediately. Everything else your dog offers you would simply ignore, during this time you would not want to speak to your dog or move. Dogs are masters at body language so if we move around while they are trying to learn we can confuse them or distract them from offering the behavior. Once your dog is reliable at this stage and is consistently offering sits one after the other you move on to the next step. Ideally you are looking for your dog to offer 10 sits in 1 minute before adding a cue.
Adding the cue is the next step in teaching a behavior, let's continue to use sit as an example. You are going to wait for your dog to begin to sit, the moment they begin to dip say the cue "sit" then click/treat when they are in the position. This is called pairing a cue, see dogs learn through associations so by saying the cue as the dog is performing the behavior they will learn to association the two together. Over time you would begin to say the cue sooner and sooner until you are cueing the behavior. You would think this is where the behavior is finished but it is not, there are multiple other steps to your dog truly "knowing" a behavior. Let's get into it!
Once you have added the cue you are going to begin to generalize the behavior, this means you want to practice pairing your dogs cue in multiple different places and while you are in different position such as sitting down, standing etc. I typically recommend my students pick two places inside of your house to practice and two places outside of the house. During this stage you are beginning to train in different areas, which means your dog might be distracted which is okay. If your dog is not offering behaviors in the new location go back to the basics and begin to capture. Capture the behavior in the new area until your dog is offering it reliably and then begin to pair the cue. Generalization is a key factor to your dog "knowing" a cue, dogs do not generalize well on their own so this is something we have to teach.
But wait theres more! Once you have generalized the behavior we then need to take it 3 steps further to truly proof it. First we introduce duration which is where we teach a dog to perform a behavior for a period of time such as holding eye contact or holding a position such as sit or down. For this you would cue a behavior such as sit, and reward your dog over and over every couple of seconds for holding the sit. Your dog needs to be able to perform a behavior for a period of time before you can move on. Second you introduce distance, this is where we teach a dog to perform a behavior at a distance from us. To practice this you are going to tether your dog to something sturdy stand back a couple steps and ask your dog for the behavior when they do it click/treat and repeat. The final step to this is teaching your dog to perform the behavior with distractions. Start with something easy such as having someone make kissy noises while you cue the behavior, when your dog does it click/treat. Gradually add in harder distractions such as tossing a toy, have someone clap their hands, or even ring the door bell.
Once you are able to cue a behavior in any environment, with duration, distance and distractions, then and only then does your dog "know" a behavior.