Looking at your passport (old or new) should be enough. Passports usually include stamps for every trip.
I suppose that depends on what passport you carry. There's a whole lotta trips with my US passport that have no stamps, esp. between the US and Canada... Same for Europeans moving between other EU countries.
And most of my passport stamps are quite illegible.
So I'm glad I missed this form, it would've been a nightmare for me.
For future reference - at the moment anyway - it looks like this form is part of our future too - to renew the PR card every five years, and for those who intend to pursue citizenship. So a word to the wise - start keeping a record of all your travels, you're going to need it.