Well, when the pump fails it's going to be really hard to steer the truck when it's barely moving - you're going to need some Hulk arms. That's the biggest giveaway. Any power-assisted vehicle can be steered safely without the power-assist; that's a safety feature that allows you to keep control of the vehicle if you lose power. So if your truck is running fine, but you need Hulk arms to move the wheels from a standstill or a crawl, that's a pretty good sign.
There's not a lot to pumps - they either pump or they don't. If you feel it getting harder to turn the wheel at low speeds, it's possible the seals are starting to go and the pump is losing efficiency.