My cat had mineral oil for hairballs. What are toxicity signs?

Mineral oil is unlikely to cause aspiration pneumonia if it is swallowed normally by the pet. If it's vomited and inhaled then, yes, it certainly can cause serious illness, including potentially aspiration pneumonia, however at this point there isn't much that could be done by a veterinary professional unless she has vomited and is showing signs of pneumonia (coughing, breathing difficulties, lethargy, etc) which won't develop for at least 24 hours after vomiting and inhaling. Making her eliminate the mineral oil by inducing vomiting is not indicated, since this will (obviously) increase the chances of aspiration.

To answer your question, "How will I know if she's OK?", you'll know she's OK if you don't see the symptoms described by Dr. Anstaett below. If you don't see any of these issues, you'll know everything is fine. And in the future, the recommended product (Laxatone) is a much safer way to encourage hairballs to pass.