Not sure how old your child is but let's say your child is 2 years old. Take the current height of your two year old and then double that height.
If your 2 year old is 34 inches tall then they'll likely be around 6 feet tall when they reach adulthood.
You can also look at how tall current relatives are now that are adults to get a prediction or how tall the kids dad and mom are to predict height.
But there's no sure way to know but you can predict the height your child might be when they get to adulthood.