How can Rh- women tell if a man is also Rh-?

The evolutionary benefit is obvious:

Rh- women likely have some form of ability to detect in men what would have been existential knowledge especially before Anti-D shots were available or the Rh factor was even known about.

Can women smell the Rh factor in men?

There is a reason for everything and rather than being stuck in the belief strategy, searching for further answers has been helpful.

Can rh negative people recognize each other? Part II

If you have read through the comments of above posts, the following may have stood out:

1. The unexpected finding that Rhesus (Rh) factors, including the Rh blood group antigens (RhCE and RhD), are distantly sequence-related to microbial ammonium transporters hinted at a possible unidentified biological function for these proteins. Now Biver et al. report that mice lacking the renal Rhcg factor have impaired ammonium excretion and a reduced ability to deal with acid stress. Rhcg is also required for epididymal fluid homeostasis, with clear consequences for male fertility. This work contradicts the common assumption that ammonia transport across the collecting duct epithelium occurs solely by lipid phase diffusion, and points to a role for Rhcg in ammonium handling and pH homeostasis in both the kidney and the male reproductive tract.

It doesn’t have to be the blood mice smell but could be whatever smell is associated with related factors.