Very rough rule of thumb (with lots of exceptions):
Igneous rocks are almost always safe
Metamorphic rocks are usually safe
Sedimentary rocks are best avoided.
The acid test is frequently used, but isn't that reliable, especially if you are only using vinegar.
Be wary of anything with metallic colors/flakes/bits
that said, rockyards, garden centers, landscaping supply places can be great places - you'll get stuff much cheaper then at an aquarium store.