I continually find ammo packs on the ground and I will be low on one type of ammo, but when I go over to it, it tell me my ammo is full. How do I tell between the different types of ammo lying around so I know which ones I need and which ones I don't need to run over to try and pick up?
Ammo packs on the ground will refill some percentage of all of your weapon's ammo, regardless of type. They will refill ammo for weapons that you've got holstered, as well as your active weapon. The exception seems to be certain "heavy weapons" like the Krogan flamethrower.
I haven't worked out exactly how much they refill, but when you get the "Ammo Full" message, all your reserve ammo is full on all of your weapons.