How can a skinny guy with a fast metabolism gain muscle? how could i track if i'm gaining any muscle?

Muscle gain: You can increase your protein intake with branched chain amino acids. This needs to be done along with weight training. Make sure you are eating a balanced diet and not just carbohydrates due to the fast metabolism. A fast metabolism is a catabolic state and will break your muscle down, so you must consume enough calories and nutrients to avoid this phase.