Serum IGF BP-3 (Insulin Growth Factor Binding Protein-3) is the more reliable test of GH levels. It is down stream from GH (except for in the liver, where it is independently produced), but has a longer half life. If this is normal, then your GH should be normal.
Dr. Mark Gordon tests for this to check natural GH output
Somatomedin C (IFG-1) is more reliable than testing actual growth hormone, but the half life is still pretty quick and therefore not as reliable as IGF BP-3