Currently you cannot peek inside a queue to look at entity attributes. As RyanG said, you can only pull out attribute values as entities enter or leave the queue.
With that in mind, you can put together some blocks that will cause all entities in the queue to leave the queue, and be fed back to the same queue in a feedback loop. Use a signal to open a gate or something that triggers this whole feedback loop. As each entity goes round the loop, pull off any attribute values you care about and send the entity back into the same queue.
This scheme will work if you want to iterate over ALL entities in the queue. If you just want the attribute value of the last entity that entered, or last entity to leave the queue, Ryan's suggestion would be better. Use a Signal Latch to save the attribute value before the entity enters the queue.