R' Eliezer Melamed, in an essay about this blessing on the Beit-El Yeshiva website, formulates the standard thus:
חכם מחכמי אומות העולם שידוע כחכם וגאון באחד מן המדעים, כלכלה, מתמטיקה או אחד ממדעי הטבע, ובעבודתו תרם תרומה נכבדה למדע ולאנושות
One of the scholars of the [other] nations, who is known as a scholar and a genius in one of the sciences, economics, math, or one of the natural sciences, who through his work made a significant contribution to science and humanity.
I think that this standard would not include philosophy students who don't produce science and would also not include expert technicians who know a great deal but haven't contributed to the body of scientific knowledge themselves.
He adds that the scholar must also be a follower of the Seven Noahide Laws, for:
וחכם שלא הצליח להבין את ערכן של המצוות היסודיות הללו, אין ערך לחכמתו, ואין ראוי לברך עליו (שו"ת מנחת אלעזר ח"ה ז, ד).
If a scholar hasn't succeeded in understanding the value of these basic commandments, there is no value to his scholarship, and it's not proper to bless upon him. (Responsa of Minchas Elazar Volume 5, 7:4)
My translation and link.
That would put Nazi scientists right out, as they clearly placed insufficient value on the Divine commandment to not murder.
That's still a pretty amorphous standard. If one isn't sure if a particular scholar qualifies, R' Melamed says, in the entry in the online version of his Peninei Halacha series that covers this blessing (15:18), to say the blessing, omitting "are You ... King of the Universe" ("בלא שם ומלכות"). (He cites Tzitz Eliezer 14:37 as saying that due to such doubt, one should only say this blessing nowadays without that part, but says that most authorities and the state of the practice is not so. It seems to me that he's adopting the Tzitz Eliezer's opinion for cases when there is a doubt.)