@pandabear, by the texture and shape of the bottom leaves in particular. Scroll to the side to see some other photos of it. You can also tell by how their leaves feel if you are new. Not that you want to disturb the powder on the leaves much. But they feel plump and harder when not dehydrated.
@pandabear - my best advise for you now is a moisture meter. That way you can visually see the plant as well as the confirmation of the meter. I used mine all the time when I had the big community pots. Not so much now that they are in their own.