Change your pump every 75,000 miles with an OE or OE-equivalent. If your pump is old but has few miles, whether or not you should replace it with a new pump will come down to how lucky you feel.
Periodically check your pump for play by removing the belts and fans. But at that point, you just might as well replace the pump. Keep in mind once you remove a pump that's been in place for years, you usually will destroy the seal.
So have new seals on hand if you plan to re-use a pump. No matter how robust a pump is, it's still a good idea to replace the seals while you're in there replacing the rest of the components. (like for example if you ran a "lifetime" stewart)