There is a healthy weight range you can look up on web sites like the WebMD, the CDC's section on health, the National Institutes of Health (NIH), etc. The better charts will take into account your height, build, and age.
BMI is also good, but allows for a relatively wide range of healthy weights. I'm 5'9", 131 lbs., and still (barely) within the healthy range in terms of BMI.
Good luck identifying a healthy range for yourself.