Most dungeons are cleared when you have killed all important inhabitants, and as a result of doing that, you have seen the whole dungeon. The game doesn't force you to loot stuff (e.g., Werewolf form can't loot) so it comes down to just killing; you don't have to do your quest.
A victorious sound will play once you clear the dungeon, and the ambient sound will stop.
If you don't kill all important inhabitants, inhabitants might respawn and the dungeon will not be seen as cleared. Another occasion where inhabitants respawn in a dungeon is when you start a new quest that involves that dungeon; however, it won't change the cleared state of a dungeon.
There are reports that some dungeons don't get cleared, or are cleared a different way. Some report that whatever they try to do, the dungeon doesn't get cleared; this might be because they still have a quest to do in the dungeon. Other report that a dungeon gets cleared after looting a chest or learning a new dragon shout word. Please note that not every dungeon has a word wall.
I've found the best way to make money is pick up ingredients in the wild, make potions and sell them.
Easy to find
Wild ingredients are easy to find. Or you can buy ingredients from many vendors. Avoid buying expensive ingredients like void salts, fire salts, daedra hearts etc. The return isn't worth the cost.
They're light
Most ingredients weigh little. If you make a lot of potions and can't sell them all, the potions also don't weigh a lot - 100 potions weigh about the same as 1 piece of heavy armor. Their value/weight ratio is high.
They sell for a lot
I regularly sell a 0.1 weight potion for $200. There are other items with higher value/weight ratios, but I can quickly make 10 of these with ingredients that aren't hard to find. This works best for invisibility, because you can sell it for squillions. I've made around 20k mostly from potions.
I have seen potions with values in the thousands!
It's good for your alchemy skill
It'll also help your alchemy skill, and as your alchemy skill gets higher, you'll make more valuable potions.
You'll always have potions
There's nothing worse than getting into a battle without the right potions. Having a few spare potions for health and magika on hand will help you a lot!
Win win win!
Best Answer
There is currently no way to enchant detect life onto an item, but there is a spell you can cast that constantly drains magika while you are using it. There's always the chance a modder will add it in at some point.