I think the problem here is that OPhiARen is very familiar with NPHC groups and is in a position to make automatic connections between activities and NPHC membership. While I'm not sure you could stop ANY member walking down the street, you'd notice signs such as colors, symbols, or (clearly) letters. Even being involved in specific service.
However, you're very unfamiliar with NPC groups, so you wouldn't necessarily be aware that the woman with a lavender and maroon tote decorated with hearts is likely to be a Sigma Kappa. Or that the college girls volunteering at a Ronald McDonald house have a better than average chance of being ADPis.
My campus has sororities that serve: Girl Scouts, gerontology/Alzheimers, Ronald McD House, Campfire USA (at risk girls), and helping the blind.
I bet that most of the NPC women on here can identify which sorority is which simply from one single example of each sorority's philanthropy.
I'm very unfamiliar with NPHC groups. My campus does not recognize them. Saint Louis has city wide chapters that do have events on SLU's campus as well as at Harris Stowe, WashU, etc. The only thing I know about Sigma Gamma Rho is that their mascot is the poodle and they have a sort of pageant/fashion show thing once a year. I see signs. But without letters, i wouldn't know one from an Alpha Kappa Alpha. I have no idea what
This is simply a difference in what you are surrounded by and what you choose to surround yourself with. It's not an issue of NPHC having identities and NPC not. It's an issue of awareness of those identities.
Phew, sorry so long, hope it all makes sense
Edit: Also NPC doesn't really "market" or promote itself to high school students. At least we're not supposed to. It's a difference in recruitment procedures.