I think that's an awesome question, because both you and your dog really do know the difference between learning a behavior, and knowing a behavior, what gets a little more complicated are the differences between prompts and cues.
For simple behaviors, like a sit, you can probably add the cue pretty quickly. You lure the site, lure the sit, lure the sit. Lures are prompts-hand signals can be prompts or cues depending on what information is being given. A prompt gives the dog hints, while a cue a specific behavior that goes with a specific word. See the difference?
The dog is concentrating on that bit of food going up over his head. Now in one training session or several smaller sessions throughout the day your dog has gone from six or seven times of concentrating on the food going over his head to seeing the prompt and putting his butt to the floor, with enthusiasm and without hesitation because he KNOWS when the sees that prompt and puts his butt to the floor he's going to get the treat. Now it's time to add the cue, the light bulb moment has occurred, and I feel if you get good at training you will start to see that light bulb moment, although in some dog's it's more obvious than others.
So YEAH it's time to add the cue! But, what does that clue really mean to the dog? The cue might mean something like "when I hear the word sit, with my human standing in front of me, with a treat in her pocket (I smell it I know it's there) and she raises her hand up over my head while we are in the bedroom, I put my butt to the floor."
And that is why when you raise the criteria (distance, distraction, duration) in any way you need to start from scratch. Go from the bedroom to the kitchen (a more distracting area of the house for most dogs) and lure the sit, wait for the light bulb moment (should come faster this time) and the behavior to come consistently, then add the cue. Now the cue means to the dog "When I hear the word sit, with my human standing in front of me, with a treat in her pocket, and she raises her hand up over my head while we are in the bedroom OR the kitchen, I put my butt to the floor."
And so on, and so forth until the dog understands that sit means butt to the floor no matter where I am or what I was doing, and I sit until I'm released to do something else.
Sounds like a bit of a pain. Sometimes in dog training you might be temped to skip certain steps, but in my experience skipping this step just leads to later stalls and setbacks, and everyone frustrated.
If you asked for a sit 100 times, and for whatever reason your dog sat 5o times but the other 50 times he doesn't sit when you ask for a sit the 101st time, what do you think the chances are the dog will sit? 50/50, right? So now let's say out of those 100 times you asked for a sit the dog sat 100 times. Now what do you think the chances are that the dog will sit when you ask them the 101st time? 100%, right? THAT is a reliable command.

Don't mix up your reinforcement history for the cue, make it meaningful and you'll have that reliability.