How can I find out more about this stone and be sure its not a synthetic?
Most gemstones described as synthetic alexandrite are actually synthetic corundum laced with vanadium to produce the color change. This Alexandrite like sapphire material has been around for almost 100 years. The material shows a characteristic purple-mauve colour change. The stones will be very clean and may be available in large sizes. Gemological testing will reveal a refractive index of 1.759 - 1.778 (corundum) instead of 1.741 - 1.760 (chrysoberyl). Under magnification, gas bubbles and curved stria may be evident. When examined with a spectroscope a strong vanadium absorption line at 475 nm will be apparent.
However, the most assured way to find out if you have genuine alexandrite is to submit your stone for testing to the Gemological Laboratory. Here some links:
American Gem Registry, Inc. http://www.gemlab.us/
American Gem Society Laboratories http://www.agslab.com
Asian Institute of Gemological Sciences http://www.aigsthailand.com/
European Gemological Laboratory USA http://www.eglusa.com/
GemResearch Swisslab http://www.gemresearch.ch/
Gubelin Gemmological Laboratories http://www.gubelinlab.com/
Russian Gemological Server http://www.gemology.ru/
Gem Testing Laboratory of Great Britian http://gem-a.com/