Most rocks are usually safe, IME. I don't think there's anything you can do to test the rocks in your tank without removing them, unless you want to wait until they've potentially affected your pH level. However, most rocks or pebbles that have been rounded by water or time are usually safe. Quartz is safe. Amethyst is safe. Granite is almost always safe. Slate is safe. Petrified wood is safe. Any rock that is extremely soft (you can break or crumble it with your fingernail or an eating utensil) is generally not safe. I'm sure others can add to the safe/not safe list.
