Identifying intervals by ear is the foundation of ear training. This is necessary to develop the highly trained ear that you want. To do this, you must memorize each interval's sound.
Every interval has a specific feeling and sound. And one of the best ways to train your ear o identify intervals is by singing. If you can sing an interval, you deeply understand how it sounds because you can generate it. And a very effective way to train your ear to do this is by singing scale sequences.
A scale sequence is a pattern in which you play a scale. The most popular way to play a sequence of 3rds. This is where you play the 1st scale degree and then play up a 3rd. Then you go to the 2nd scale degree and play up a 3rd from there, and so on. To learn more on how to do this, watch the video below.
Now that you understand how to train your ear to identify intervals put this exercise on your practice schedule and do it often. But this exercise alone won't get you the desired result. Training your ear takes a lot of practice using many different ear training exercises.
If you want to learn more about how to develop your ear the fastest way possible, click the link below.
https://eartrainingforguitar.com/complete-ear-mastery-course