Most avalanches occur on slopes between 30 and 45 degrees. You can measure the slope angle with an inclinometer, or you can "eyeball" it by dangling a ski pole by the strap and estimating the angle.
Before you cross a slope where there is any possibility of an avalanche, fasten all your clothing securely to keep out snow. You should loosen any pack you have so that you can slip out of it easily and remove your ski pole straps. Make sure that your avalanche beacon is on and switched to "transmit" rather than "receive." If you are with a group cross the slope one at a time to minimize danger.
What to do if you are caught is an avalanche.
The first thing to do is to get off the slab, which is not very easy.
If you're descending on skis or snowboard, try heading straight down hill to build up some speed, then angle off to the side off the moving slab. If you're on a snowmobile you have the advantage of power. Grab some throttle and use your power and momentum to your advantage. If you're headed uphill, continue uphill. If you're headed across the slope, continue to the side to safe snow. If you're headed downhill, you're only hope is to try and outrun the avalanche. Remember that large avalanches travel 60-80 mph and they are difficult to outrun.
If you can't get off the slab grab a tree or thrust your hand upward. When avalanche debris settles it settles like concrete so any indications of where you are (like thrusting your hand upward out of the snow) must be done before the avalanche settles.
Collapsing snow, cracking snow, and weather will all alert you to the likeliness of an avalanche.
The many mountainous areas in North America have an avalanche center that issues regular avalanche advisories. This gives you an easy, overall view of snow stability for your area.