Some signs of low leptin from my experience:
get cold easily,
hit fatigue earlier in workout (less fat mobilization = less stamina),
never sweat at night (nightsweats = active thyroid, I think),
less morning wood (in my experience),
desire for food > desire for sex (this is definitely not right),
eating less with no weight-loss (T3 correllated with leptin)
To check your metabolism, take your temperature in the morning before getting out of bed. Also, get some blood tests that test for T3 and T4.