Can i use the rate at which my facial hair grows as a proxy to see how strong my level of human growth hormone is, roughly speaking?

No: Growth hormone is not responsible for hair length or rate of growth. Facial hair growth is more attributable to sex hormone secretion from the pituitary like fsh/lh and testosterone levels as a feedback loop of the testicle, adrenal, and pituitary gland. Growth hormone is more responsible for skeletal growth and height.

Not exactly: I don't think the notion you are raising has been looked at from scientific standpoint. Although its true that growth hormone is influential in hair growth, there are other growth promoting factors such as estrogen, thyroid and testosterone hormones. The hormone that can closely correlate with endogenous (or exogenous) gh levels is Insulin like growth factor 1 (and to a lesser extent igf-bp3).

HGH and Hair: Hair growth results from the interaction of several hormones on the skin. The most important factor being andgrogen levels. So it is not a good indicator for hgh levels in the body.