1889 & 1890 Morgan Silver Dollar Coins - What Determines Value The Most ?

Hey guys new here. So my brother and his girlfriend have acquired a small coin collection, he let me take a look at it the other day, and a couple coins caught my attention one because they looked in such good condition but also because of their age. They were a 1889 and 1890 silver dollar. When I was a kid I dabbled in coin collecting but I really don't have a clue about the matter, aside from little bit of online research.

So these coins are fully sealed in a case and I couldn't see the backs of them, which my understanding is very important for these coins if nothing else for the mint markers. I don't think they wanted to take them out but later when I told them these coins in-particular the 1889 could potentially be very valuable that changed things. When they are available again I'm going to take pictures of these coins and un-case them so I can evaluate them more. But until I can do that I thought I would ask some experts on the matter about what to look for to determine the value?

From what I been reading the condition is very important of these coins it's hard for me as a novice on the matter to really say how "good" they looked to me but by looking at these pictures on the grading site here https://www.PCGS.com/grades the coins from what I remember looked at least like the PR-66 grade. But also like I said I know the mint mark plays a huge role in value with the CC carson city mark being the best. I here about proof versions being valuable too but how much ? And how can I determine if either of them are proofs ? I know more modern proofs kind of have that high contrast effect on them. Also I know there is a rare "bar" error mark on some that is valuable like beneath the eagles bottom left side of the coin.

Anyhow any help on figuring out better what to look for on rarity/value for those years of the coin would be helpful. I'll post pictures of them when I can today or tom.