will saves

[Surpriser]

On a semi-related note, can you actively tell if a spell someone casts on you is Will negates [Harmless] versus Will negates?

You can, by making an appropriate Spellcraft check (DC 15+spell level if you can see/hear somatic/verbal components or DC 25+spell level after rolling the save).

For the OP's question:
The one guy who succeeded on their saving throw feels that a spell was cast on them (and failed). They can then try to identify the spell (but not the caster, as there where no visible signs of the casting) with a DC 27 Spellcraft check.
Of course, there might be other ways of identifying both the spell ("Hey guys, that dude likes to wear pink underw...I shouldn't have said that!") or the caster (the guy who just entered through the door is a pretty obvious suspect)