The cleric has the magic domain.

Each of the three casts Melf's Acid Arrow.

My question is:

1. Can a sorcerer/wizard/cleric/bard watching these three cast pick out which is the sorcerer, the wizard, and the cleric? Do they cast the spell differently?

2. Can a non-casting class tell which is which by how they cast?

Thanks.

Don't take this as policy yet but -

Without asking anyone else, and with a vague recollection of another similar question that was asked in this very thread, I don't think you can, simply because there's no mechanical way to tell. You'd just log out, look at their 7 cleric levels, log in again and say "ahah, I knew it" which is metagaming really.

Just my opinion though, the real guru's out there'd know better than I.