All posts that have had new posts since you last read them have a green arrow, like this
or this
if you've answered it.
To find all the new posts since you last visited RYL, go to "Search" [on the forum's toolbar] and then "find all new posts". That will show all the threads that have the before mentioned green arrow.
To find all the threads without any posts, to go "Search" and then "Advanced Search". It's pretty self explanatory from there :)
Hope that helps!
Chelsea x