Glass is known to be a generic term that can be used to describe the material itself. Crystal is known to be one type of glass that is available. This means that a crystal is usually created just like how glass is created. The main difference is the type of materials that will be used to make crystal and glass.
One thing that you can do to differentiate if you are looking at glass or crystal is by trying to place the glass and crystal near a light source. If the light source will cause the item to have a rainbow prism effect, this means that you are looking at the crystal.
If there is no rainbow prism effect that will take place, this means that you are just looking at the plain glass. Another thing that you can do to test what you are looking at is the sound that they will make when you try to tinkle. Crystal will have a more musical sound.