I'm 20, how to check the status of growth plates in respect to increase height?

X ray. A single x-ray of the left wrist and hand is usually all that is required for a "bone age study". The results are compared to standard xrays of kids that age and gender to come up with the observed bone age. Open growth plated are readily evident.If you are 20, most are likely closed.
X ray of left hand. By simply asking your physician to order an x ray of your left writ and hand (so called bone age). This will determine if you have any future growth potentially. If there is significant delay, you may ask for referral to an endocrinologist to make sure your hormones (thyroid, growth, cortisol) are all within range good luck.