Different types. Physical exam is most important. Bone age x-ray to determine "biological age". If this looks like true precocious puberty to a pediatric endocrinologist then it's important to determine the cause. A stimulation test will help identify if the pituitary is releasing pubertal hormones because random levels can be misleadingly low. If puberty isn't pituitary generated, tests for other causes needed.
Bone age. Bone age is first test to be done. It is an x-ray of wrist.
Precocious puberty. In early puberty hormonal screening tests may not pick up elevated levels. The best "tests" are history and physical exam. In girls breast buds are the first sign and boys it's testicular enlargement. An x- ray of the left hand and wrist called a " bone age" can discern whether the growth plates are closing too soon.