It is a perfectly good practice to check the wing by your self but it's not always possible. Most places we deice the BAe-146 we do it on deicing stands with engines running, so going back and opening the back door to check is not an option! We have to trust the ground staff when they tell us that the deicing of wings and tail is completed.
And by the the way, type I fluid does give you a little bit of hold over time.