Well, once my 50+ year old Saniware toilet broke this week, I suspected and eventually discovered a crack on the bottom of the tank. Wanting to inspect the other hidden corners of the inside of the tank, I used the camera on my iPhone to inspect! (After washing out te inside REALLY well!). Holding the phone upside down in the top hole (with toilet removed) I snapped four different pictures in camera 90° each time. Worked really well and have a good view of the interior of the tank.
Thought I'd share in case someone else has the same issue! :-)