And that's a good gague - how they react in new situations. A dog with a 'wait and see' attitude isn't necessarily weak (in fact it's a trait of our breed). I would suggest that they have different types of nerve / temperament. But neither is weak.
A 'weak' nerved dog would have a very hard time adjusting to a new sitaution. Their reactions are often exhaggerated (aggression, bolting, hiding, etc.) and extreme.
If you read through a description of a Temperament Test
http://www.atts.org/testdesc.html you'll see that the dog can be startled. Like a "Whoa! What the heck was that?" but then must recover and show interest "Oh, that's all it was."
At one TT, a woman had a mixed breed dog. Gun shots (bang, pause, bang, bang). The dog startled and bolted to the end of the leash and continued to dart around. The dog did this for a full 20 seconds AFTER the shots. There was no recovery - only flight response. An extreme reaction where it wasn't warranted. That dog failed - weak temperament. She had mentioned she had the dog since it was a pup, so he wasn't abused.