Through just knowledge of the game or being able to use corsix. Ask me any unit in the game and I'll tell you its armour type. Brits and PE generally have soldier armour. Units that can take several sniper shots (KCH/Lts) have heroic armour. Rangers, most vet2/3 axis infantry, guards etc. have elite armour. Airborne have airborne, snipers have sniper armour duh... The rest usually have infantry armour.
Infantry armour is the basic armour type, with no modifiers generally. Soldier armour takes much less damage from enemy bullets, especially BARs and other special weapons. Elite armour takes less damage and is harder to hit than infantry armour. It takes more damage from flames and snipers cannot miss it, even while retreating. Heroic is like elite armour but takes much less damage from snipers. Airborne is like soldier, but additionally is harder to hit while the unit is moving.
Elite armour is one of the most important concepts in the Wehr/US matchup. Grenspam is so powerful because grens change their armour from infantry to elite at vet 2. This makes them take 25% less damage and are 25% harder to hit. Obviously this is a huge benefit. Against vetted grenspam, you have to use artillery, flamers, snipers and tanks to do real damage because riflemen are just too weak against the strong armour.
