I believe monitoring your strength level is better way to gauge your leptin level.
Assuming that you have been holding all your dietary variables and recovery variable constant, at hypercaloric diet, you know that your leptin level is "relatively" low when you start gaining mostly fat rather than muscle, despite heavy lifting. Similarly, when you cut, you lose mostly muscle rather than fat when your leptin level is low.
How can one know if one is gaining muscle or not? Easy -- you just monitor your strength gains/losses over a week or so.