Are you a Tarantula hobbyist? If so, we invite you to join our community! Once you join you'll be able to post messages, upload pictures of your pets and enclosures and chat with other Tarantula enthusiasts. Sign up today!
You can tell severely dehydrated tarantulas because they will curl their legs under them and/or have a tiny or shriveled abdomen, but by that point it is a very sick spider.
Offer a small water dish so you don't reach that point. Most of their moisture will come from their prey.
I would add a lot more substrate to that enclosure, and use either peat or topsoil without additives. Get it at Home Depot for dirt cheap. If you want to mix in some of that bark, you can, but it isn't necessary. Basically, add enough substrate to fill that tank about halfway.