There are lines that show going toward you from any PC or NPC taking any action (with a cast time? Not sure if instant actions show) on you or another party member. Heals will show as blue lines coming from the healer to whoever they're casting on. I believe there's an option to turn this off, but it's on by default.

There's also the healer's cast bar in the party frames that shows what they're casting, though not who they're casting on. So if you can catch both of those, you can maybe catch when it's happening.

Realistically, though, just get used to overheals. By the time you can catch what's happening, if you can, the window to make a decision has probably passed.