1. Check the headphone jack and the dock input...many people who just do a quick external swap of the cases from black to white won't take the time to do these parts, and they'll still retain the "black" models pieces...here are pics for comparison...you can see the white inside both, the black one did not have white in either, and would be a pretty good sign it was swapped vs. stock white.
2. Like he said above, registration is the easiest way of telling exactly how old it is, because even with Apple care, there is still a very precise date for when the warranty started and ends.
3. Not really, outside of fully charging it up and seeing how long it lasts under your normal usage conditions, there's no real way of testing the battery flat out.
4. Again, like he said above, try a SIM card of a company that doesn't carry the iPhone in your market, if it works, you're set.
5. Here's where i disagree a bit from the first response you got, he said to make sure that the original box and all that, and to avoid sellers with several iPhones for sale...no real reason to count out sellers without the original box, my wife is one who never keeps boxes to stuff, but her iPhone is in mint condition, and if she sold it...a person who would avoid it because of such would miss out on a great phone for probably a really good deal (and if you're doing so to avoid getting a stolen iPhone, understand how easy it is to get a box for an iPhone, so if you ARE going to require a box, make sure the serial #'s match)...also, sellers with multiple phones can be a good source of a deal as well because the truth is, people buy bulks of these things all the time, and sell them off...no real reason to assume shady business, again, you could miss out on a good deal.
The bottom line is you just have to be careful with what you buy no matter WHAT you search out...i had a lady one time try to sell me a chinese knock off with the original box for a REAL iphone, even the phone itself looked real, but when powering it up, it was very obviously a fake. You have to use common sense and make sure you know what to look for on the phone...
- Condition of the exterior that meets or exceeds your expectations
- Menu systems (NONE of the knock off iPhones are even close...not one of them, so it will be very obvious from the get go just looking at the menus and apps)
- Go into settings and check the "About" under general settings, look for a legitimate firmware version, model # and serial #...those things can be verified online to ensure you're getting the real deal.
...good luck! Buying an iPhone 2nd hand isn't rocket science, it just takes common sense and some patience.