Many people have advised going for a test walk carrying a full rucksack but I would also add that it might be worth kitting yourself out in your waterproof gear and standing under the shower for five minutes or more. I bought ponchos that said "100% waterproof" but turned out to be as leaky as sieves.
Of course, if you are going in hot summer weather this won't be important as you will be able to dry off - but if not . . .
Of course, if you are going in hot summer weather this won't be important as you will be able to dry off - but if not . . .