You should flush your plants as part of your watering schedule. Based on my experience, one flush once every three weeks - use 3x the volume of your growing container. Discard the runoff, although you can do things like check the pH of it to evaluate the soil conditions.
"Signs" you need to flush are indicative you have waited too long. You'll get pH fluctuations because of salts, which will lock out nutes. You might see signs of under-fertilization, fan leaves turning lighter, and patches of your plant turn unexplainably a lighter color than the rest of the plant.