ok good question… i definitly meant to touch on this… but the post got lengthy and i got bored – one of the biggest questions in the game is criticals…
excuse any punctuation/spelling… i’ve had a few beers and some delicious wine –
crit….(insert ammo here) happen at random… or so you think at first. Till you start to read some of the changelogs in the beta and realize… well then – not so random after all…
let’s go with the most obvious and frustrating.. or sexy 
– crit-rocket – in general, a crit does roughly double damage, as far as i can tell and is signaled by the pichew sound, and a red or blue coloring.
now i’ve had some time alone on the wcu tf2 server and i’ve looked a lot of things, this being one of them – this is what i’ve concluded…
when you first spawn beginning of a map, i’d give a crit on any given class about a 12% chance, every 8th rocket, at least one in 2 clips as a scout… 1 demo sticky – you get the picture – test if, it’s pretty accurate i’m sure. math was a strong suit –
now… with that said, the basis is 12%…. we move onto some special circumstances, which can be found by reading all the changelogs and inferring some stuff…
when you’re on a roll, you crit more often, i’d say up to 35%, meaning – you kill 4 or 5 in a row, you notice more crits… rename to butter….
when your on your deathbed – ie. less than about 25 hp – or so that i’ve noticed, crit % seems to raise to about 25-40%
And then also i haven’t seen this written anywhere, but i firmly believe location on the map deals in a bit also, i’ve noticed more crits happen in the enemies territory than my own. I can’t confirm it anywhere, but i’m fairly sure it’s true.
oh and btw crits are in every class in every weapon… except sapper i’m pretty sure. woot heavy crit (uppercut ftw)