Usually, it's a good idea to replace it if it's discolored or looks grimy. You won't feel it in the steering or get warning lights, but you can pop open the cap and check the fluid color. If it's not looking good, it's probably doing slow damage to the metal and rubber parts in the steering system. I did the flush on a Lexus SC430 recently, and it's the same basic procedure on all cars. Easiest way is to get all the fluid out of the reservoir using a turkey baster or transfer pump, then refill with fresh fluid. Then drive for a bit, and keep repeating this over the course of a few days. Once you've gotten through 3-4 qts of fluid, you know you've replaced about 80-90% of the fluid which most people say is good enough.
Or if you're like me and want to go for the challenging way, you start with some hose, (about 6' of 3/8" i.d. hose from Home Depot would do), some hose clamps, a 3/8" coupling and a 3/8" bolt. Cut about 2" of hose and plug one end using the bolt and a hose clamp. Then undo the return line on the power steering line (the one that does not connect to the p.s. pump, you can trace it down to the steering rack or the cooler). In place of the return line, insert the 2" blocked line you just fabricated, and hose clamp it down. You've blocked off the return port on the reservoir. Next, use the 3/8" coupling and hose clamps to extend the return line from the engine bay to a drain pan located in a suitable place. Then, get in the car, unlock the ignition and turn the wheel lock to lock a half dozen times. Don't start the car since you'll be starving the p.s. pump and doing some damage. This should help evacuate most of the fluid from the system. Then you put everything back as it was, and fill fresh fluid in the reservoir. Start and let the engine run, then refill the fluid. Ignore the groaning/moaning coming from the pump as it primes.. You may need to recheck and refill the fluid a few times, but after that, you should be good. Hope this helps FWIW...
