Wanna peek without mods and without cheats ?
Then read this, this will help you very much, hopefully will work on every source game.

Start up your game, its very important !
Go in the console.
Type in "bind [Leaning key] +alt1;-alt1", this is left lean.
Now your first key is done, second is this:
"bind [Leaning key] +alt2;-alt2", this one is right lean.

Now you have a tactical advantage over your enemies being that you can lean, but you cannot shoot with your gun, that wouldn't be fair wouldn't it ?
They cannot see you leaning, but you'll be turning to the side you're leaning, it can be really useful to see where is the sniper exterminating your team, see where the enemies are so you can throw your 'nade right on 'em, or just spy on people without them seeing you.

I'm not sure if this work on every game on Source, but it at least works on Gmod.