Agreeing with Knerd and Bartuc.

As anecdotal evidence...
At 5'4" and ~107 lbs, that puts me just at "underweight." Which is honestly laughable. I'm certainly pretty lean, but no one could look at me and accuse me of being underweight. I have thighs so thick that buying pants that fit them (while not being too big in the waist) is pretty damn hard. I don't count calories, I eat dessert every night, don't exercise, often overeat, and am in the habit of snacking even when I'm not hungry. The only sort of "diet" I have is that I'm vegetarian - and that certainly doesn't preclude me from junk foods. I don't have ribs sticking out, my face isn't gaunt, my hair is healthy and my eyes are bright... What I'm saying is that my weight is healthy (even if my eating habits need a little work, but not in the way "underweight" makes you think ).

Really, the only reliable way to know if your weight is healthy or not (unless it's glaringly obvious) is to just talk to a doctor. They will take things like muscle mass vs. fat into consideration, as well as all the other variables in the equation.