you see a sword appear above the hit target if it crits and you will also hear a sharper noise when you crit during judgment.
When judgment crits, it only crits the Attack Damage part of the skill:
Damage dealt: 15/35/55/75/95 + 35% attack damage
With 100 AD, a normal hit = 130 damage
a crit would be 165 damage.
Also, Infinity edge and "crit damage runes" apply damage to this crit.
So with infinity edge + above numbers you get: 182.5 damage.
Every .5 seconds every target is hit seperately and crit chance is also seperate (an attack from 1 rotation can crit on 3 enemies and not crit on 5 others at the same time).
The best way to build Garen is to go Tank/DPS hybrid. Early game, you can get away with going DPS only (youmuu's ghostblade), midgame you will want to start going towards a more tanky setup (e.g. Atma's Impaler, Phantom Dancer, if you fight a physical team), for late game you will need pure tank items (guardian angel, force of nature, etc.)
Garen is one of those champions that is not defined by his skills; he has good damage output if build for it and is still able to take a beating, or you go for barely any damage but he then becomes an almost unkillable tank. As a pure tank though, the problem is his ultimate; it's a killsteal pretty much. So personally, i advise the DPS/tank hybrid route, using your ultimate to quickly gain gold, turning you into both carry and tank for your team.