Centr trainer Ashley Joi has a certified method for determining that, you could call it the talk test.
"If you can still talk, then you are working but not to the point where you are to your full capacity," she told Women's Health. "So, every once in a while you want to get to that point if someone asked you a question, you have to look at them like give me a minute and then I can say something."
But she stresses that you don't have to spend your entire workout in that state (because, RIP).
"If you can get to that point, at least one to two times in your work out... There's nothing wrong with being at a point where you can talk, but when work is really happening, everything should be shut down and all your focus should go towards what you're doing, and that's the difference."
Fortunately, a little bit of HIIT goes a long way so you won't be huffing for too long. Research has found that three days of HIIT a week is enough to burn fat and build fitness, according to one study conducted by The University of Western Ontario. When participants did four to six 30-second treadmill sprints (resting for four to six minutes in between each sprint), they burned way more fat than another group of participants who slogged it out doing steady-state cardio for 30 to 60 minutes three times a week.
Plus, a 2014 study found that those who did HIIT were more likely to enjoy them over longer, lower-intensity options, that might have something to do with recent research revealing the enormous endorphin rush it offers. So. Many. Perks.