Has had regular oil changes... noted oil very low. Added oil. How will I know if there's damage done? Prior the car was making a fine sound with acceleration...now its quiet. I noticed there's a little oil leaking underneath.
Very low oil won't necessarily cause damage. If oil gets low enough to cause damage, you won't be asking how to tell if there's damage, your engine will likely seize. Meaning smoke billowing out from under the hood and loud metal grinding sound and the car won't start anymore.
Most oil dipsticks are super conservative, the dipstick can be dry and you still have just barely enough oil to operate the engine. For most engines, the oil pressure warning light will come on when oil gets really too low.
Do find where oil is coming from exactly. It's likely an engine seal or the oil pan gasket. Get it looked at. :)