How do I determine if I need attic insulation to keep cool in summer?
What kind of insulation do you currently have in your attic? (Note: head on up there and check it out.) If it's batt insulation -- large units of fiberglass -- then that information will be printed on either the paper facing or the insulation itself. If it's loose fill insulation, which is far more common in attics, then you can usually figure out what the product is (pink, yellow, or white and very itchy = fiberglass, dirty grey = greenfiber, white and not very itchy = cellulose) and find it's value per inch on this handy table of R values. 

For best performance, I'd go up to R-60. In a home built by your average builder in the past decade, R-30 meets code. Adding blown-in insulation, which you can do over fiberglass batts, is extremely cheap if you DIY ... although it's not a pleasant job to do. Rental of the machine from Home Depot or Lowe's is usually free with the purchase of a certain number of bales of insulation, and greenfiber especially is cheap. To bring my 1600 sq. ft. ranch up to R-60 would only cost me a couple hundred dollars in materials. 

Do check at all ends of your attic. You might find that the areas close to the attic access are R-30 to meet code, but areas farther away might have less. I've known a few builders to be stingy with the insulation. One home I helped a friend with here in Houston had less than four inches of insulation in most of the attic. 

One caution point is to make sure that in the process of adding insulation that any attic ventilation that you may have at the soffits is not blocked off. Soffit air is essential to the ventilation of your attic, if your home is designed that way. I would recommend installing Berger AccuVent baffles over every soffit vent before adding more insulation. 

Last but not least, consider coating your roof with a reflective coating. You might need to get a contractor to do this, but it's simple and straightforward work. In the Inland Empire, nothing is more effective than keeping the solar rays from heating your house in the first place. Unfortunately, many find these roof coatings to be unsightly, so you might have to check with your HOA if you have one before applying anything.