There are certain early signs of MBD (or other vitamin deficiencies) that might help you notice. Eye closing/squinting can be anything from feeling bad to a vitamin or calcium deficiency. Swollen legs can also be a sign, just rule out any infection first. Poor bowel movements, tongue shooting, shaky feet, grabbing himself without immediately correcting, having issues moving limbs correctly, all good signs to look for, for any possible deficiencies or sickness in general ^^
And that's how you'll become an amazing keeper. Those who love to learn do the best in my opinion with chameleon care. But don't be too hard on yourself if you make a mistake. We can only be so perfect until something happens.