Zircons have vastly higher birefringence than topaz. You can see this by examing the blue gem with a 10X loupe. Be sure to examine it in all directions, since the birefringence of zircon varies from strong to weak depending on what angle that you view it.
Go to a jewelry store or to some friend with a known true topaz. Each back facet of the gem will be doubled and will be parallel when viewed at 10X. If the doubled facets are very close to each other, it is topaz. If they are VERY widely spaced, it is zircon. It is very easy, for even an untrained person to see this, if you have a known blue topaz for comparison.