After doing some research I could not find anything indicating how to know the proper inflation. I understand the pressure should be 3psi. But how does one know when this pressure is reached?
About inflation pressure, the working pressure is 3 pounds per square inch. Every foot bellows pump that I have seen, except some specialized pumps for high-pressure Zodiac floors, can only inflate to about 3 psi (that is, unless you perform a circus act by jumping onto your foot pump from a high platform).
Therefore, inflate your boat as much as you can with your footpump, until the inflation becomes inefficient. Then reach down and squeeze the chamber with your hand to register what that correct pressure feels like. You can actually judge pressure changes within about 10% just by feel.
There are a couple of obvious conditions that will alter the original inflation pressure. Anytime the boat heats or cools from the original temperature when you inflated it, the pressure will change. So if you inflate the boat on a hot beach, and then put it into very cold water, the boat may get a little soft. This is no threat to the chambers, of course, but you may want to top off the boat when it has adjusted to the cold water. Conversely, let's say you inflate your boat on a cool, cloudy morning, then paddle for half a day, hauling out onto a hot sunny beach with no shade -- then the tubes will heat up and the pressure will increase above 3 psi. The boats are individually tested at 4.5 psi, so there is a safety margin, but I recommend venting a little air from the chambers while you are having lunch as a precautionary measure. If you take your foot pump with you, then you can top off the kayak before resuming paddling.
By the way, we do sell gauges that work like tire gauges. You stick one end in the open valve and read the pressure on a dial indicator. You get insignificant air loss with this gauge.