Hmm. I may be able to assist in ascertaining this...
...goes back what feels like a thousand years to anatomy and physiology...

Gents, could I suggest a thermal camera app to your iPhones? If the object of your affections is ovulating her body temperature will be mildly increased.
(Medical_Male Say aaaaaaaaaaaaaaaaaah, Tue 15 Dec 2009, 17:58, 2 replies)

Or she might be in the secondary stage of syphilis.
Just saying.
(Juan Quar is clinging to her front teeth on, Tue 15 Dec 2009, 18:10, closed)

Very true.
Alongside rashes on the palms.
(Medical_Male Say aaaaaaaaaaaaaaaaaah, Tue 15 Dec 2009, 18:19, closed)

I see
Excuse me love would you mind putting this in your mouth?
Hands over a thermometer
(insomniac-surfer oh go on then, just this once, Tue 15 Dec 2009, 18:34, closed)

May Be Easier?
www.amazon.co.uk/iCAMView-Infrared-USB-Camera-CM-01d-01/dp/B001IUDQRA/ref=sr_1_2?ie=UTF8&s=electronics&qid=1260902558&sr=1-2

It's an IR camera ... :)
(Medical_Male Say aaaaaaaaaaaaaaaaaah, Tue 15 Dec 2009, 18:43, closed)

Hmmm
I think being introduced to someone who then produced a IR camera would freak me out
(insomniac-surfer oh go on then, just this once, Wed 16 Dec 2009, 2:03, closed)

yeah, but it's near IR*
it will just give you a picture that looks like a funny coloured version of an ordinary camera, as it will only pick up IR frequencies near the visible spectrum.

You want far-IR to get proper heat tracking. The stuff that's heading down towards radio wave frequencies.

*I'm assuming this, based on the £18 price tag.
(Spidge New sig needed, donations gratefully accepted., Wed 16 Dec 2009, 12:02, closed)

Bacon Rashes?
yum!
( Jimbuktu See? Right here, Tue 15 Dec 2009, 21:56, closed)

And you would know this how?
;)

Edits
pfft that was a reply to Juan up there
(insomniac-surfer oh go on then, just this once, Wed 16 Dec 2009, 2:06, closed)

Or
The pub/club or whatever you are in is packed and steamy
(insomniac-surfer oh go on then, just this once, Wed 16 Dec 2009, 2:05, closed)