Check the front of the rear wheel wells. The seatbelt mounting points are prone to rusting out. There is a rectangle in the wheel well, smack that with a hammer too. The factory undercoating will hide severe rust in there, forgot to mention that.
Luckily I found the rust in mine before...
Doesn't look too bad to me. I would scrape/pressure wash away that black goopy undercoating and see what it looks like underneath it. smack the frame with a hammer in several spots. Hit it with some force. If it's severe rust you will punch through or knock chunks off