You can do it manually from the properties tab by right clicking in the game list.If something goes whacky with your game, this sometimes fixes it.
Hope I helped a lot of others too : DI have this problem, I think we're boned until valve launches steampipe as The Qless mentioned.Here's what it looks like: Both were free and both download updates incessantly, so I uninstalled them.I've been aware of the validate steam cache files for a while.I was to do an clean one without the taskbar, but I couldn't get as transparent. If you, for example, you shut down computer with steam turned on, it will not work and will validate cache again. So the game must keep the caches from you equal from the steam servers. So: The PC can't BSOD / The Steam can't crash nor the lights from your house depletes it.
Search for tf2 validating:
Every time I try to play tf2 it tells me it's validating my steam cache files, and then stops at 100%. I was using Avast for a while and TF2 would have to validate every day or so.