As of a fairly recent patch, the random dungeon finder no longer tells players in queue whether a dungeon is in progress or not before you confirm you are ready to enter the dungeon.
However, once you click accept and before you are teleported to the dungeon, you can see a role check of people who are ready. Players who are already in a group are automatically set to ready, this means that by watching the ready or not ready statuses of other group members, you can make an educated guess on whether the group is in progress or not.
Heres how to exploit these mechanics:
1) Join the dungeon queue while you are in a rested area such as a main city
2) Open your ingame menu where it allows you to log off
3) When you are asked to join the dungeon through the ready check, click accept (the faster the better so other players cannot react)
4) View the ready statuses of each member, you tend to find that if a dungeon is in progress, there is usually two people or more that are instantly ready.
5) If you see more three or more question marks on the ready status, it is likely that the dungeon is not in progress.
6) Quickly press log out if the ready statuses are not to your preference then log back in, you are safely removed from the queue and you can queue again.
Granted, this does have a bit of luck in it as some times DPS that have been queueing for a long time can click ready before you, but its a rare occasion and you usually can abuse this trick quite easily with a short queue time of tanks and healers.
Enjoy farming those easy points and getting some easy call to arms bags!