Hi Gary, welcome to the forum. some of my suggestions below..
Determining Real or Fake Watch - my opinion only
For the newer pieces, you should be able to get a new one from an authorized dealer fairly easily. But if your friend / a seller is letting the watch go for a great price that cannot be passed up, or it is an older piece, then I think the 'genuine or not question' can be answered in the following ways:
a) Does it have box and papers ? If they do, good - but this is not a guarantee that your watch is real, although the papers should indicate where it was sold, and the warranty card should have the casing number written on it, which should match the casing number of the watch.
If it does not have box and papers, then ask yourself if you trust the seller, and does he have a good reason for not having box and papers, and where and when did he get the watch from ? so this here is 'buy the seller, not just the watch' - i.e. does your instinct and senses tell u the seller is trustworthy ?
b) the dial and the case back can be authenticated visually - google for images of the case back (although there are great fakes out there). So this here is the visual authentication, much dependent on one's ability to determine genuineness in the metal or in the pics.
c) size and weight - if you have access to the workbooks
www.weeber.biz/IWC/Workbook/search.html (older models you can find it here: www.weeber.biz/IWC/Workbook/2007/3255_s.pdf)
you can use measurement tools to measure and weigh to ensure it is according to specs. perhaps a bit extreme, but I do it myself, especially on the weight - I am not beyond bringing an electronic balance to check the watch's weight. the workbook says, for example, the ref 325502 weighs 71 grams, measures 39mm and is 11.5 mm tall, and lug size is 20mm. I suppose if it is a fake, (hopefully) at least one of these details will be wrong.
d) try setting the watch - adjust the hour and minute hands; slow adjust the date; quick adjust the date; check if the hacking seconds stop when the crown is pulled to position 2. this ensures the movement & complications are as according to published specs, and also checks for the working condition of the watch.
e) if a service center is nearby, would the seller be willing to go with you and have the watch authenticated there?
I am guessing it would be through the case and movement number of the watch (and the solid caseback - or any caseback for that matter - will have to be removed by an experienced watchmaker).
f) If you pay by paypal or credit card (possibly - depends on your arrangement with your credit card provider), there is an avenue for dispute if u do not get a real watch. So here: possibility for getting your money back through a payment 'buffer', but note this is not full-proof.
my 2 cents.