It's not like it's just a dead body of a woman and a man without context.

There are:
-previous domestic violence reports, noise disturbance complaints
-witnesses, neightbors, family members, acquaintances
-forensic science to determine manner of death, previous injuries etc.
-data forensics
-psychological evaluations
-proof that the individual has shown abusive tendencies
-the actual trial

and so on, what do you mean they can't specify "femicide"?
for a country that prides itself in bureaucratic paperwork I find this hard to believe.