The safest way to address someone in the SCA is with "Milord or Milady". A person can style themselves Lord or Lady So and So only if they have received an Award of Arms from the King and Queen.
Counts or Countesses are those people who have sat the throne once and are called "Your Excellency". These folks will wear a coronet that is "embattled" and gold (looks like a castle top).
Dukes and Duchesses have sat the throne twice and are called "Your Grace". They will wear a coronet with strawberry leaves on it.
Viscounts and Viscountesses have sat the throne of a Principality. There are styled "Your Excellency" as well. They will wear a coronet that is embattled but it will be silver.
Court Barons and Baronesses are made at the will of the Crown for service specifically to them in their reign (for the most part). They are styled "Your Excellency" as well. They wear a silver coronet with six shperoids.
Landed Barons and Baronesses hold lands for the Crown. You must have 50 paid members in a certain land area to become a Barony and the group chooses the Baron and Baroness and the Crown grants them the lands and titles. They are styled "Your Excellency" and wear a gold coronet with six spheroids. Louisville and Cincinnati are Baronies.
Knights are referred to as "Sir". This Order of the Chivalry is limited to authorized fighters and Knights are recognized by the Crown.
Masters and Mistresses of the Laurel are those individuals who have excelled in the Arts and Sciences and are recognized by the Crown. They sometimes wear a medallion that displays the Laurel wreath.
Masters and Mistresses of the Pelican are those individuals who have given great service to the Kingdom and are recognized by the Crown. They sometimes wear a medallion that displays a pelican "in her eity" which is a Pelican feeding her young her own flesh pulled from her breast (yuck).
All of these titles are earned in one way or another. No one can come into the SCA and proclaim themselves a Knight or Duke or Master, well they can but people will make fun of them and probably not want to hang out with them.
So like I said, it is always safe and proper to address someone as Milord or Milady when you are not sure and then ask someone later. It took me a long time to get it all straight and most people
don't mind what you call them as long as you are polite.
**note** Baroness Morgan wrote this for me over a year ago so I wouldn't totally embarrass myself when addressing others (lost cause I'm afraid). I hope she doesn't mind my sharing it with others. T.