- You think about the characters a lot outside of the story and can happily envisage a continuing life for them.
- You can think about a sequel (or indeed prequel) and the prospect of this pleases you! (Better still you get on and write them!).
- You find yourself recalling what Character X would do in a situation you face and knowing you've got it spot on. You may well find their initial reaction is better than your own!
- You find yourself wanting to write more for and about them.
- You continue to build up a picture of what they look like, possibly using real pictures to help here.
If you are finding your characters encourage this reaction, then brilliant. Your characterisation is strong, they really are seeming like real people to you. If not, flesh out your characters more. Drip feed information telling the reader what they like and. Show us what they dislike and so on. Good luck!