Gaining fat VS Gaining Muscle

BlackBeltInTraining

Hello everyone! Sorry to bother you all, but I have a quick question.
After being on this site for about one and a half weeks I have gone down from 166 to 164, but for about two days I was at 163.4. I know I shouldn't worry about the numbers, but if I am doing something wrong then I want to be sure that I fix it before it gets worse. So, I was wondering, how do you tell whether you are gaining fat or gaining muscle? While I'm at it, I also wanted to ask if this weight could possibly be water weight. Currently, I work out for about an hour a day, doing martial arts as well as running a few miles. My calves are becoming much more toned, but I'm not sure about the fat /muscle ratio.

Thank you so much for your time and help!