Camping trip disaster - tent flooded at 2am

So we're at Lost Lake near Portland with my 7yo and 5yo, thought we had everything figured out. Pitched the tent on what looked like flat ground, but apparently there was a slight slope. Woke up to everything soaked. Wife's sleeping bag is completely ruined. We packed it up and drove home at 3 in the morning with two very confused kids. The drive home was actually the best part because they both fell asleep. Anyone have tips for checking ground before setting up? We're not giving up on camping but I clearly need to do better.