The first test that has always been used for these gems is refractive index - flame fusion (Verneuil) spinel has RI 1.728 versus natural 1.718 - small but very consistent. Flux grown spinels have properties that overlap the natural but you don’t usually see these in large sizes. Also, Verneuil boules will possibly show the curved growth lines called “striae” which are proof positive of synthetic and if you are lucky this will show up with a penlight through the stone in the right orientation. The likelihood of finding a 45 carat blue gem outside a museum in my opinion is slim to none, and I am leaning more towards the “none” side.