The method for determining exercise intensity by estimating it without measuring it is called rating of perceived exertion(RPE). In this method, you will estimate a number from a scale of 6(no exertion) until up to 20 (maximal exertion).
You can also try talk test but it more qualitative. In this method, if you can sing that means the exercise is light, talking is medium, if it just word is high intensity.