Best sensor to determine "excited" ?

First thought was to buy/build a pulse sensor (little thing on your finger using an IR and IR Receiver and an op-amp) to detect minute blood flow...

Second thought, using your bodies skin (the more sweat the more conductive you become) to determine if the person is excited or normal.

Third thought, using a piezo speaker in reverse, via an opamp or transistor and attach it with a band near to the heart....

Basically, i'm trying to gain the best method to try and determine if the person is getting excited, for example "I bought you a lava lamp" i'm not trying to determine a lie... but instead determine if the person got "excited" at the thought, i doubt very much someone would get excited, but if i said to you

"Here's $10,000 and a ticket to travel anywhere in the world anytime you want"

I'm expecting some kind of "measurable" change, unless it's Mike I think every sensor may fail with him.

This is one of those times it would be useful if people had tails. Then you could just check for wagging. XD

I woke up this morning (OK, afternoon -- it's Saturday) to my GF telling me she was going to buy me a cupcake. I wasn't plugged in to an Arduino, but I'm pretty sure that I was excited. I friggin love cupcakes. :D