Snowpack is affected by temperature variations and load. Your job is to find weak layers that could avalanche under your weight.
STEP 1: MATCH THE ASPECT
Perform your test on a low-risk slope that has the same aspect as the one you'll ski. This will give you the best picture of the snowpack. Dig your pit six to eight feet deep and four feet across, leaving a smooth, flat wall on the uphill side.
["CUT A COLUMN"]
STEP 2: CUT A COLUMN
Working with the uphill wall of your pit, draw two vertical lines on the slope about 20 inches apart. With the tail of your ski, isolate a column in the snow (use a sawing motion), leaving the column in place.
["LOOK FOR LAYERS"]
STEP 3: LOOK FOR LAYERS
Examine the column you've created. You'll probably see several layers. Brush the snow with your fingertips. Does it crumble or is it hard? A soft layer of crystals under a slab of consolidated snow is a recipe for a slide.
["DO A TAP TEST"]
STEP 4: DO A TAP TEST
Place the back of your shovel blade on top of the column and tap it until a layer shears off or until the column collapses. The number of taps it takes for the column to fail is one of many go-or-retreat signals for backcountry skiing.
STEP 5: EVALUATE
Now think about your findings to determine where and how the failure occured. Did a four-inch slab of soft snow release at the top? Or did your column fail cleanly two feet down? When I see any instabilities, I hesitate to ski that slope. I'd rather ski something I feel good about than drop in with fear and have somebody dig me out—if I'm lucky.
Disclaimer: As A.J. mentioned, the shovel shear test is only one of many snowpack-analysis techniques. Take a class, and practice your avy skills before you head out.