How do you check your blind spot before changing lanes?
Turn your head and shoulder-check: mirrors alone do not show blind spots
Side mirrors show only narrow angles, so turning your head to do a shoulder check is the only way to be sure nothing is in your blind spot. Check your mirrors and then shoulder-check before moving over.
Why the other answers are wrong
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Glance at the side mirror only; it covers the blind spot
A side mirror covers only a narrow slice of what is beside you, and the blind spot is the part it misses. Check your mirrors first, then turn your head, because the shoulder check is what actually confirms the lane is clear.
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Rely on the blind-spot warning light
A warning light is a helper, not the rule being tested. Blind spots differ by vehicle and can hide a whole car, so the handbook counts on your own shoulder check.
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Listen for other vehicles before moving over
Your ears cannot tell you what is beside you, and a cyclist rolling past your rear corner makes almost no sound. In most vehicles the blind spots sit back left and back right, so you clear them by turning your head and looking.
Quick way to remember
Mirrors alone leave a gap; your neck does the rest.
Based on the official Ontario MTO Driver's Handbook.
Reading it once is not enough
Drill this question in a scored round and find out what else you are shaky on.
Practise Rules of the Road: The Basics