A great question - took me more research than I expected!
First, to tell opal and uranium glass apart, you can measure the specimen's specific gravity. Opal can be up to 2.25 g/cc (Wikipedia gives a range of 1.9 - 2.1 g/cc for hyalite opal). Uranium glass is denser, with a range of 2.49 to 2.57 g/cc. Glass is also likely to have bubbles in it.
For an idea of the value, the opal listing in the price guide gives ranges for different qualities of crystal opal, and the opal buying guide and article on appraising opals can help you determine its value.
Geologist and IGS Team Member