I would wait until the dog is two, but there is a standardized test (in the US, anyway) by which you can get some answers about your dog's propensity to defend you in a controlled atmosphere.
It's called the American Temperament Test, and details can be found at
http://www.atts.org/testdesc.html
Basically, the test simulates a walk (on leash) during which you encounter all sorts of unexpected stimuli, from somebody opening an umbrella, to shaking rocks in a bucket, to shooting off a pistol in the air, to walking over different texture surfaces, and neutral, friendly, nonthreatening, threatening and aggressive strangers. You're not allowed to give the dog any feedback with the leash, just stop at the various points and see how he behaves/reacts to what happens there.
The neutral stranger is just a place where you stop and exchange words with somebody who ignores the dog completely. They want to see if the dog settles down and relaxes, eyes them suspiciously, etc. The next person you greet greets you AND the dog, and the way the dog reacts to meeting a strange friendly person is noted.
Later on in the test a peculiar dressed person crosses your path (some distance away). He's just weird, but not engaging you at all - the purpose is to see if the dog notices him and in what way if any the dog reacts.
Then the weird stranger starts to approach (this is all done at quite a distance and at no time is your dog ever in any position to get to this stranger) and act threatening. He's probably wearing a raincoat and hat and waving something around and yelling gibberish.
At this point the dog's reaction is judged.
And then the threatening stranger really moves closer (again never closer than about 15 feet, I think) and the dog's behavior is noted.
I found it a really valuable tool and I will seek it out in the future to confirm what I already believe about my dogs. Keep in mind that breed is kept in mind -- you don't expect a spaniel to react to a threatening stranger in the same manner as an Akita. Also, in the gun shot test, they told us that hunting breeds are permitted to look to the sky for a bird, but that our boxers are expected to look back at the source of the noise.
It's not merely a test of courage (more of self confidence, actually), but how APPROPRIATE the dog's behavior is when it comes in contact with different unexpected events. A dog may react quite boldly/aggressively to a stranger approaching you on leash, but be afraid to approach a pail of rocks that have been shaken and set on the ground. That would tell me that the dog is acting out of fear, not confidence. I have found that most boxers I've seen don't show too much outward sign of concern about the stranger until he starts actively acting aggressive.
My favorite part of the temperament test day was watching my friend's french bulldog valiantly charge the threatening stranger (at least to the extent the leash allowed). Throughout the test he was as manly and brave as any dog ten times his size.
) but she is also a scaredy cat..I don't think she realizes how intimidating and big she really is.