Is there a way to predict who will have an adverse reaction to letrozole?

Skin testing... Can be suggestive of an immediate allergic reaction, but is not a validated test. As far as other adverse reactions, including delayed allergic reactions, there is no predictive test which I am aware of.
No. Letrozole is an oral non-steroidal aromatase inhibitor for the treatment of hormonally-responsive breast cancer after surgery. As in all medications of this type there are several side effects but i know of no way to predict who will get which one.