You just test if the pH rises (means it's not inert). Testing GH and KH can work too. The more acidic your water is, the more noticeable the change is (dissolves stone faster). If you pH and hardness is already high, it might not show much difference.
Another test is pouring something acidic on the stone (out of water) and see if it fizzes. The more acidic, the easier to see the reaction (fizzing), but too acidic can be dangerous to handle. White vinegar (acetic acid) people commonly use, but it's not that strong of an acid. Muriatic acid is stronger, but caution is advised when handling.
Stones that raise the pH/KH and GH aren't that troublesome, but good to know what you are dealing with. Hard, alkaline water does put your water chemistry in that range, so noting specific livestocks water parameters are wise. Can't healthily keep a soft, acidic water fish or invert in hard, alkaline water (vice versa).
Coming from reefs, just think of non-inert stones as crushed coral and the likes.
Know what substrate you are using?
