I'm car shopping too. The best way to tell is often the photos of the car are clearly not taken anywhere near our latitude. Trees with no foliage are a sign that it's a stateside vehicle. VI plates are also a good clue, although some people don't like to show their plates so they remove or obscure them.
The decision to ship a vehicle from the 'States can be complicated, but there are some things weighing in favor of that option.
Used car prices are usually lower stateside, and roads being better, a stateside car will be in much better shape for equivalent mileage.
You'll pay road tax (.16/lb) on a car you ship, but smaller vehicles don't weigh so much and the lack of used small crossover suv's, for example, means your car will jump up in resale value enough to cover the tax and at least some of the shipping costs. With any luck you can maintain that value until you sell.
Excise tax applies to some vehicles, so if you can settle for a vehicle with a VIN starting with 1, 4, or 5, you won't pay any excise tax.
Edit: This article is informative, in case you haven't seen it. And I would contact Paradise Freight if you plan to look into shipping a vehicle to STX.