Couple options here... if the riddle has been unlocked from interrogating the Riddler thugs (if you see the green square brackets on the map showing that there is a riddle in that area), then the riddle should stay on the screen so long as you are in the correct area.
Otherwise, (I think I have seen it come up on the screen briefly before I've interrogated a Riddler thug) you may be able to see it in the Riddler part of the menu. (From the map screen, right bumper over to the "?" square). This is all the Riddler items you have can (or have unlocked), what you want to do is hover over the logo that has a square bracket with the person inside (these represent the riddles) on the menu and if the riddle is available it'll have it listed here (this menu is sorted by area so find the area you are in and hover over the riddle). If it reads "This riddle will be revealed when the time is right", you're probably better off just playing through the story until it unlocks.
So, you can just beat up and interrogate Riddler thugs until the riddle is unlocked, or give the menu a shot and try to find the riddle for the area you are in (I tried finding screen shots for the menu screen with no luck, hopefully you know what I'm saying!!).