Did i get a deal on my ring ?

i bought a pre-owned diamond ring at a well known jeweler. gorgeous to me..5/8 carat center diamond and a total weight of 2 carats. i have seen this ring on the website for $4700.00 new. of course i know there are different grades or whatever they are called. I paid $1900.00. how do i know if i did good or not ? it could be a cz and i wouldn't know.