FYI, my wedding ring has no marking, for religious reasons. I've also had some rings resized, and lost the marks.
I've found a LOT of gold and silver that isn't marked. You start to get an eye for it. One thing that may help is to go to a big department store, and look at the real jewelry and look at the fake. When you have both at home, put them together to see the differences. I usually wear my 14K rings on my left hand (engagement, wedding, and anniversary bands). I will usually wear something that is 10K on my right hand. You can put a piece of jewelry up to the rings to compare. If it's cheap and you are willing to risk it, go for it.
And about the costume, yah, some pieces are AMAZING! I got a box that had a bunch of Avon in it. I tripled my money on 3 brooches, and still had a ton of good stuff left.
Sometimes when I can't fall asleep, I do searches on sold items on ebay .... vintage rhinestone brooch, avon brooch, lots, etc. Whatever interests me. Sometimes I do a search for silpada earrings or bracelets .... you never know what you will see one day and find IRL a week or two or three later. Just learn to file it away in your head.