How can you tell the difference between serum and plasma after a specimen is centrifuged? How can you tell that a patient has a high platelet or white blood count by looking at a centrifuged plasma specimen

I don't know that you can tell by looking what the hematocrit count is...but I can tell you that serum is yellowish looking and plasma is dark red colored after it has been centrifuged.  I guess if there's lots of yellow and only a little red, we could safely say the person is plasma 'low'.
Hope this helps.
G.