Each Nepenthes will generate a different number of glands with different activity levels in each of its pitchers. The largest pitcher of my N. sanguinea barely produces liquid at all, just barely enough to drown a few insects and digest them. Other pitchers on the same plant produce liquid up to 1/4 or 1/3 the pitcher in depth. Some pitchers produce a huge amount of nectar, dripping all over the table, and some produce only enough to dry on the pitcher itself.

No, your plant does not "feel" hungry or full like we do, so cannot salivate like many animals do. It will react to environmental cues, like insects struggling in its pitcher, by generating more digestive enzymes, but this is just a simple reaction that is not learned or easily changeable and is not connected with a central nervous system.

If the plant is still pitchering and growing normally, it has enough fertilizer. If in doubt, drop an ant or other small insect (or even a tiny bit of fish food, like tubifex worms) in a pitcher every few weeks.