If he seems excited to see you ("Hey, there's the big thing that feeds me!"), then that's a good sign. Not necessarily a bad thing if he DOESN'T seem interactive, but a happy, healthy axolotl can be pretty fun to watch. My boys notice when I walk by the doorway and usually come up to the glass if I approach their tank. Sometimes one or two of them will even slowly swim toward the top if I'm nearby but not crouching down to their level.
Those are the kinds of things to look for. In terms of health, you want them to be fairly chubby (width of stomach similar to width of head) with fluffy gills. In terms of happiness, a decent sign is whether they move around and seem to enjoy the ornaments and stuff in the tank. If they're always in the same spot whenever you check on them, that's obviously not a great sign.