Watching star placement and timing is one of the better yardsticks to go by for sure and that's defo the first thing I look at when I pug alongside an AST I don't know.
The cast bar thing is the trap answer I was hoping for though. My reply is aimed at healers more casual and less skilled than you though, so please don't take offence <3
Back on track, I get the feeling that a lot of more casual healers are under the impression that if they don't see GCD cast heals, their co healer isn't likely isn't healing. Seeing nothing but nuke casts kind of seals the deal for them.
If Miraidensetsu/Luno is still here and willing to join in the discussion, I put forward the following question:
I killed Innocence Extreme the other day in 7:56. In that time I did 206 total casts. 11 were healing GCDs with a cast time with 6 of those being Medica II. Beyond that I cast 96 Glares, I placed Asylum twice, did 2 Tetras total, didn't Benediction once and lastly, had an ~80% uptime on Regen.
What percentage of the overall healing do you think I did?