I can feel it. My hair will feel somewhere between greasy and gritty, like it has very fine... stuff... stuck to it. It will also look greasy down by the roots, but a dull kind of greasy. I also definitely notice dull color -- I have auburn hair, so it's very easy to tell when it suddenly looks brown.

Just throwing this out there: my hair also gets easily overmoisturized, which does not feel greasy, but instead looks shiny and healthy - but with no body or bounce. I confused this with build-up for a while. Now I can tell the difference.
I sort of blog
I'm mostly away doing field work through mid-May.

Hair: curlybob, fine, dense, low-po, big waves/ringlets (2C/3A)
Co-Wash: Nature's Gate Herbal Conditioner
RO: YTC Carrot, Aubrey Organics GPB
LI: BS Shea Soft Styling Cream (HG)
Styling: KCCC, Garnier P&C Gel
1xWeek: low-poo, gelatin PT or Joico K-Pak
Hair Likes: aloe, coconut oil, humidity, protein, long walks on the beach