You boil it in sawdust; you salt it in glue You condense it with locusts and tape Still keeping one principal object in view, To preserve its symmetrical shape.
Lewis Carroll, The Hunting of the Snark
The snowpack
New snow
Most avalanches occur as a result of newly fallen snow. Snow comes in a variety of shapes and sizes. Much is made of the expanded lexicon indigenous people of the Arctic have to describe the states and moods of snow. However, we have just as many in English: powder, stellar crystals, graupel, corn snow, hoar frost, riming . . . just to name a few. In fact, Magono and Lee (1966) identified 80 categories of new snow crystals.
Layering
Snowfalls to the ground at different times and under varying weather conditions. The result is not a uniform blanket, but a series of layers which may or may not have similar properties. It is these layers of differing characteristics that are the underlying cause of avalanches. We only see the surface and maybe the top few layers of snow, but it can be layers of snow several feet deep that may ultimately determine whether the slope will fail.
Metamorphism
Snow crystals change over time heavily influenced by current weather conditions, especially temperature. The form and size of snow crystals and grains inside a snowpack change continuously, altering the strength characteristics of the snowpack.
If you wish to learn more about snow metamorphism on a technical level I recommend The Avalanche Handbook.
Snow Tests
Out in the field there are a number of tests we can perform on the snow to develop a picture in our mind's eye of the snow stability where we are at the time. Keep in mind that snow stability can vary from slope to slope and even across the same slope depending on subtle terrain factors.
Probing the snow with an upside-down ski pole is a quick test of the different resistances of snow layers.
Snow pits
Digging a snow-pit (a smooth vertical walled hole big enough to look in) reveals more about the snowpack structure than is visible from the surface. Buried weak layers - identified by a simple finger prod - may react to various (compression and shear) shovel tests. These involve isolating a column of snow and manually applying a load to that column. The pit and tests take about ten minutes to perform and give an indication of bonding between layers hence the likelihood of a slab release.
Effective snow tests take practice and experience. Most introductory avalanche courses will include practical sessions on snow pit analysis. Refer to your local avalanche centre. Links to some centres are provided in the references section.