The only way to tell is to take a rock, clean it very well with water only and a hard bristle brush so you get everything off of the rock. Let it dry for a few days then pour some vinegar on it. If it bubbles you know minerals will leach out into your aquarium. If it doesn’t bubble you should be fine.

Find yourself a rock, clean it up, let it dry. Pour some white vinegar on it. If it bubbles or foams, it contains calcium carbonate and will most likely raise your ph and hardness a bit. Sometimes that’s good, especially if you have ph that fluctuates or very soft/acidic water.

soak everything for a while before you put it in the aquarium. i pour boiling water over rocks or wood and let soak then repeat. it helps to clean and disinfect

Most slate/granite etc. rocks are fine. Just make sure they aren’t sharp enough to cut your fish if they swim into them (depends on what fish you have).

Keep in mind that some rocks can alter the Ph levels in your water (limestone for example).