There are several ways to quickly find out who has threat using the default UI without having to pay attention to a threat meter or threat percentage number on the target frame.
Red shows tanking threat. Green shows not actively tanking.
Tanking threat is highlighted in red. Nameplates increase in size to show transitioning threat and nearing potential tank change.
The absence of the red highlight on this nameplate signifies no tanking threat.
A red highlight around a player frame only shows the player is actively tanking something. While tanking multiple enemies this will go red with 1 or 101 enemies being tanked.
Red highlights on the target frame show tanking threat. Yellow highlights show transitioning threat and a possible tank change. The absence of a red or yellow highlight display a lack of transitioning (yellow) or tanking (red) threat.
Raid frames allow the red tanking threat highlight. Here Koshiba is tanking while Prune is not.
The raid profiles setup in the Blizzard interface menu system and all the settings to achieve the raid frames becoming useful for tanking threat.