If a decanter is lead crystal, there should be a mark on the bottom of the item. You can read more about lead crystal at http://www.wisegeek.com/what-is-lead-crystal.htm.
I can't tell if you mean "crystal" as in "made of a crystalline material" or "crystal" as in "made of lead crystal, which is not technically a crystal at all". Take it to a jeweler. There's not any simple foolproof way a layman could tell.
I assume you mean pure Lead, and I'm here to tell you that Lead is an element in the periodic table. The fact that it is an element means that it is only 1 type of atom; Lead! The atomic symbol for Lead is Pb.
Not to cheat! ;)
Silver is harder than lead, and it is less dense than lead. Lead is extremely soft - you can scratch it with a fingernail. And it is dense.
There are a number of ways you could tell when a hydrated crystal starts to become dry. It will start to lose its shine for example.
Crystal and glass are the same thing. Crystal is glass that contains lead. When lightly tapped it has a sharp ring compared to plain glass.
The best way to find out how much your antique crystal decanter set is worth, is by taking it to your nearest local antique dealer. They should be able to tell you how much it is worth and other information about the set.
I can't tell if you mean "crystal" as in "made of a crystalline material" or "crystal" as in "made of lead crystal, which is not technically a crystal at all". Take it to a jeweler. There's not any simple foolproof way a layman could tell.
You Really can't
I can tell you 1575 is only the date Bols (Made liquor) was founded. not when the decanter was made. Delft is the company who made the decanter and they are stilll in business as Royal Delft.
Take your crystal to a local jeweler who can use a probe to help you determine whether or not your crystal is a diamond.
ask it!!
Easly
A pyrite is a cubic solid and a gypsum is a monoclinic solid.
Most of the time it will. Good crystal always will. You can tell crystal from glass by lightly tapping the side of the glass, vase, or item. When you do this you will get a ringing sound from crystal and a "thump" sound from glass.
To distinguish topaz from glass or quartzes (e.g. citrine), we should observe the hardness. Topaz is a fairly hard gemstone, meaning it is resistant to scratching. It is 8 on the Mohs hardness scale (diamond is 10, sapphire is 9). A scratch test will determine whether a stone is topaz in most cases. You can perform a scratch test by taking a piece of quartz or other material with a hardness around 7 and scratch it against the "topaz". If you don't want to risk the stone scratching, even if it's fake, you can choose to scratch it on the girdle (the unpolished rim around the gem). There is a lot more information on this site: http://www.shopgemstones.com/topaz-treatments-and-synthetics.html
Crystal balls are just a transparent sphere. Usually they are made out of glass or crystal. Some legends and myths say that crystal balls are used by fortune tellers to tell the future.