The best ways to tell if you are a good singer are to record yourself and listen to it back, and get feedback on your singing. You can check your tone sensitivity and vocal range using an online test. Also, assess your stance, posture and breathing to ensure you have the proper singing technique.
At what age can you tell if you can sing?
By ages 5 and 6 most children will start to sing in the traditional sense, when they'll be able to carry a tune consistently and sing with steady rhythm; some children will develop these abilities as early as ages 3 to 4 (Moog 1976) (Sloboda 1976).