To be certain, you should:
* Consult your owner's manual: This is the best source of information.
* Look for a clear, labeled reservoir: Power steering fluid is typically a reddish or brownish color.
* Search online for pictures: A quick Google image search for "1979 Chevy El Camino power steering reservoir" may show you exactly where it is on a similar vehicle.
If you still can't locate it, a local mechanic or auto parts store employee should be able to assist you.