There is a difference between knowing that you have been the target of a spell and knowing that some particular person was just casting a spell. Indeed, you might think that the caster was someone who was openly casting something innocent, while a malicious spell-casting was being done too subtly to notice. (One reason to be a Mentalist.)
A look at RM2 Spell Law appears to have it mute on the question, but RMU is more clear on the matter (and follows RMSS in most, if not all, of this). Informational spells that affect an individual have a chance of being noticed, based on RR (and the RR is primarily for this purpose, with only success by 50 or more preventing the spell from succeeding). The willing-target text for Utility spells indicates that the target must take on trust what spell the caster is using. Given that Informational spells in general can have a very light touch (scanning for active magic, for example, is almost at the "passive sensor" level), I would say that Force spells would be obvious, in so far as you would feel some influence on your aura, though (going by the Utility spell rule), what the spell was is probably not immediately obvious, though successful observational type skill use might note a moment of drowsiness from a Sleep spell, for example. You would not necessarily have any idea where it was cast from, although if there is one stranger in a pointy hat present, you make make assumptions. Elemental spells, of course, are generally quite flashy, and those that might be considered exceptions (if a Deflection or Bladeturn field visible other than an unnatural redirection of the weapon is a question left for the GM) are not targeting individuals. It may be obvious where these originate (Bolts and Balls, for example), or they may appear at some distance from the caster (e.g., most Illusions) and be of uncertain origin.