Malgus gets a buff that indicates he's about to cast it. Here is a screenshot:
Unlimited Power Buff
I've found it easier to look for that than the actual cast bar. When I bring my healer to a FP/Op, I'm not normally on interrupt detail so I can focus on heals. But for this fight, I "focus target" Malgus just in case, and it is really easy to see when that buff appears, then tab target / hotkey interrupt.