Stainless Steel is available in many different grades, the 2 most commonly seen in lighting and ceiling fans is 304 and 316.
What is the difference between these to?
Simply put it comes down to it's Nickel content. The higher the Nickel content the better the stainless Steel. Nickel on it's own is extremely expensive, this is the main reason why you pay so much more for Stainless Steel then for normal metal.
How can I tell Stainless Steel from normal metal?
Well if you don't have a microscope and a degree in metallurgy, one thing you can try is a magnet. You will find that most all stainless steel products will not be magnetic. So when you put a magnet against it, it should just fall off. This test is just gauge and may not work all of the time.
Will 304 and 316 Stainless Steel rust?
This greatly depends on where you put it. However Stainless Steel is like any product you purchase. It will not last forever if you do not take care of it. While 316 will certainly give you the best protection against rust, it still requires the occasional wipe down to remove any build up such as salt. If you live within about 20KM of the Coast then we would strongly recommend 316 in all your outdoor fittings and fans as 304 will not last long under these conditions.