1. Being Memorable. Sounds easy but can be easier said than done. The advantage is a character can be memorable for good or villainous reasons but there has to be something about them that sticks in your readers' minds long after they've finished reading about them. Can you say something about your character that would instantly bring them to mind without you having to refer to the story? (This can be a useful test!)
2. Having a Life. Your characters have a life of their own, which may not necessarily be directly relevant to the story you're telling about them but which feed into it. For example, a character may be known for usually being a stay at home and then they suddenly go on a quest and they wonder how those who know them will react to this. The quest is the story but the fact the character has friends and neighbours who will gossip about what they're up to brings that character to life. The obvious examples here are Frodo and Bilbo Baggins from Tolkein.
3. They would be capable of further adventures. The great characters have traits and skills that would be easily transferrable to other stories about them. Your readers should be able to picture your characters going off on other adventures.
4. The characters are willing to be challenged or overcome initial reluctance to face challenges. I love stories with characters like these, partly because I think about what I (or my characters) would do if facing the same fictional challenges. This feeds into 3 above, of course. It is my experience characters like this always do more than their author originally thought them capable of and that is a very good sign.
5. Their enemies fear them with good cause. The irony here is that this can apply to the enemies fearing the hero, but also the hero fearing the enemy. A worthy hero deserves a decent villain to test them. You can also see why the fear is justified.