So relatively new heroes like reworked Urgot and Pyke with "execute" ulties have very visible indicators in form of mark and red border on enemy healthbars to tell if they can finish enemy hero with ult or not, but some older heroes namely Cho'gath, Garen and Darius with somewhat similar ulties(dealing true damage that can't be reduced by resistances, buffs, debuffs etc. effectively killing a hero with health lower than X) have no such luxury.
So my suggestions are:
1) Add said healthbar indicator on all enemy heroes for Cho'gath as his ulti deals X true damage and used almost 100% of the time to finish a hero/objecitve.
2) Add such indicator on "villain" for Garen as his ult deals true damage but only on "villain".
3) Darius is somehat tricky cuz his ult damage on target can vary a lot based on bleed stacks and whether Darius has bonus damage proc from passive or not. Probably adding such indicator on enemies with 5 stacks of bleed would make sense.