Pour some vinegar on the rocks. If it fizzes you can't use it.
Some say vinegar isn't strong enough but it works most of the time.
Be careful boiling rocks. I've heard accounts where they have exploded from doing this.
boil the rocks for 30 minutes in a covered turkey roaster. people are so scared of exploding rocks...i've been doing this for years, just don't put your face over the rocks and keep it covered, let cool a bit then pour cold water in the warm water to cool enough to handle. they aren't grenades.
For the most part if they came from a random, out of the way area in the woods or a big backyard that is okay. The reasoning is to avoid rocks that may have been exposed to chemical pesticides or fertilizers.
The vinegar/acid test is good to ensure you aren't using very basic rocks. But just because a rock is slightly basic that doesn't make it unsafe if you are either trying to achieve a harder water chemistry (rift lake cichlids, livebearers) or your source water is crazy soft and needs some buffer.
I would just scrub them well in hot running water and then maybe keep them in a bucket for a few days where you change the water constantly and keep an eye on the rocks for any fungus or other nasties. If all seems well after a week add them.
I did the same thing. Found some rocks in my back yard, tested with vinagar, scrubbed really well, soaked and rinsed with water a dozen times. Didn't boil. They have been in the tank for about 2 months now and no problems.