How can I tell the difference between gynecomastia and chest fat?

Firm/nodular vs flab. The breast tissue tends to be very firm and nodular and gritty when pinched between your fingers whereas fat tends to be softer and more compressible. I often ask patients to place their hands on their hips and contract their chest muscles or to raise their hands high over their heads to visualize the for mass that is responsible for gynecomastia. However gynecomastia is usually a combo of both!
Hard to tell. Male breast enlargement is usually due to a combination of excess breast tissue and too much fat. The breast tissue is usually located centrally under the nipple and feels a little more firm than the more peripheral fat. Treatment usually involves liposuction of the fat and direct excision of the breast tissue. You should be at or near your target weight before having this operation.
Physical Examination. Gynecomastia comes from the greek meaning "gyne" meaning woman and  "mastos"  meaning breasts.  it refers to abnormal development of large mammary glands in males. Usually, physical examination is all that is necessary to differentiate breast tissue from adipose tissue. Consultation with board certified plastic surgeons may be helpful.