Donors typing Rh neg at IS are required to be weak D typed because it is important they not be mislabeled as Rh neg if they have any part of the D antigen. So yes, if OP went to donate and was typed as Rh neg she could safely assume she really was. But if she were to be typed at a hospital or through his doctor, then no, she cannot assume with 100% certainty she is Rh negative unless they perform a weak D test, which is not routine in that setting. Some weak D's can form anti-D, as you've alluded to, but this is no small consequence and evidenced by the fact that we even care about Rh typing donors.

Push comes to shove the worst thing that can happen to a weak D is formation of Anti-D

Again, this is no small consequence, especially in women of childbearing age. In the blood bank, this is avoided at all costs. It's not really a "just" situation, it's a "try hard not to do this to someone" situation.

Also, as I'm sure you realize, just because something is uncommon does not mean it is not significant. Certain type of weak or partial D's are more common in races other than than Caucasian.

To answer OP's question, a standard blood test that consumer's have access to (i.e. at the doctor's office) will NOT test for weak D. But is there clinical reason to worry about weak D and go searching for it in a pregnant woman? No.