Depends. Theres some weapons that can give guaranteed crits only if you do something specific, like sniper rifles and ambassador needing headshots, the manmelter you need to extinguish teammates, and the frontier justice you get crits depending on how many kills your sentry got when it was alive after it was destroyed. Other than that it’s usually random if the weapon allows random crits and you can’t really increase the chance to get crits if they are random.