I just know what it looks like, but you got me thinking, "how do I know". Here's what Dr. Google says;
Galvanized metal typically exhibits a dull gray or silvery-gray appearance with a distinct spangled pattern, which is a result of zinc crystallization. This spangled pattern, also described as a crystalline-like structure, is a telltale sign of hot-dip galvanizing, a common method of applying a zinc coating to steel. While appearance is a strong indicator, laboratory testing is the most accurate way to confirm galvanization.
I asked the old guy at work (retired from his profession as an engineer - something in metals) how to make it rusty as I thought galvanized steel didn't rust, but of course my "rusty" brain has forgotten what he said to do.