[TheDopp]

or compare your backpack to the allitems list.
http://www.tf2items.com/allitems.php
takes a whole 2 mins of time

2 Minutes versus a quick parse of the backpack contents on page load, displayed as a collapsible checklist in a third tab next to Loadout. Or as part of Loadout, specific to each class.
I think it's a good feature request and something I'd use. Something I learned the hard way when coding for people other than myself - just because there's another way to do something, doesn't mean it's the laziest way for the users. And users like being lazy.