Well, if no one else is gonna post, I might as well toss something out!
I found these two articles that deal with recognizing (PP)D in someone else:
Some things from my experience with depression (not post-partum)...
1. No one likes to have their feelings disregarded as an imbalance of brain chemicals. So if your DH thinks it's time to have the "I think you have PPD" talk, he should consider waiting until a time when you're fairly calm. (If it's feasible, of course.)
2. Many people are resistant to the idea of sadness being treated as a medical issue. If you think your DH might feel that way, explain that just because sadness is not always caused by a medical problem does not mean it can't be a symptom. As an analogy, sometimes when you vomit, it's purely situational, an not an illness, e.g. you got on a roller coaster after eating three chili dogs. But sometimes when you vomit, it's a sign of having the flu or something. Sometimes you're sad for situational reason, e.g. your dog died, but sometimes the sadness can be caused by the various substances within us that cause our emotions.
3. Likewise, just as it is possible to have the flu without ever vomiting, you could possibly have depression without being sad. It's just one symptom of many. It seems like depression just takes all your emotions down a notch. Something you'd be happy about is now something you feel neutral about, something that should be mildly annoying is now a big problem, something that would normally make you sad will now crush your spirit completely....I think if everything in your life except the PPD is abso-freakin-lutely perfect (hypothetically), your PPD will just make you feel "bleh" instead of sad.
4. And just because you have something "real" (situational) to be sad about doesn't mean you don't have depression... but it does make it harder to recognize.
Does your DH normally know when you're sad, or are you the sort to go mope in private? If the latter, maybe it'd be a good idea to try getting in the habit of being more expressive?