If you have a refractometer and a hydrometer, you can use the discrepancy to estimate alcohol. See http://www.ncbi.nlm.nih.gov/pubmed/19323723
The main formula to examine:
Alcohol = 0.833 (apparent refractometer brix) − 0.996 (apparent hydrometer brix) + 3.927
As for it being safe, I wouldn't...