Do you know when something/someone is not a valid target for a spell?

[erok0809]

Say you're trying to use charm person on an NPC, but that NPC is really a silver dragon using alternate form to look like a humanoid. According to the SRD, the dragon keeps its original type and subtype, so she's not a valid target for charm person. When does this become apparent to the caster? Does the spell fail and you lose it, or can you try to choose another target if there is one, or can you stop casting the spell and choose another action?

[Flickerdart]

It is important to note that you always select a target after (http://www.d20srd.org/srd/magicOverview/spellDescriptions.htm#targetorTargets) you finish casting. So there's no "choose a different action" here - you've already cast the spell, it just needs somewhere to go.

The spell also doesn't fizzle - it's not "if the target is a humanoid, do X." It's "the target is a humanoid. Do X."

The rules don't seem to consider "you chose an illegal target" to be a possible case. Therefore, I posit that it is impossible to choose an illegal target; your DM must tell you it is illegal, and then you must choose another.