What's the difference between chest fat and man boobs?

Gynecomastia: You are referring to gynecomastia in the latter. Fat is just simply that, fat. Gynecomastia involves an actual breast bud forming which has a more firm texture, actual breast tissue. Your mom can help you tell the difference. We see gynecomastia transiently ( for 6 months to a year) not uncommonly in teenage boys related to the relative proportions of changing hormones.

Fat vs breast tissue: Chest fat is common and when it becomes excessive, may simulate the appearance of gynecomastia. Gynecomastia refers to a combination of fat AND excessive glandular breast tissue. While a small amount of glandular breast tissue is normal and may become prominent in over 60% of pubertal males, it tends to resolve in nearly 80% of men upon completing their puberty. Persistece may require excision.