I mainly rely on experience on a particular tank, and when I observed something, I then sometimes resort to test kits.
Some signs are very obvious, such as change of growth rate of plants, duckweeds, spot algae... etc, or things like when plants start pearling, or colour of leaves... etc.
Eheim pimp club #48
800L 100% Flourite
80L plant/shrimps nursery tank inconnected with the main tank
Total 600W HQI, 2 X 5200K 2 X 6500K
Eheim 2028 X 2 2080 X 1
Dupla Substrate Heater
Dupla S external reactor
Aquamedic 2hp & Teco R15 400W chillers
Sega pH Controller
Light period: 6 hours on each bulb across 12 hours
Water Parameters: pH 6.4, KH 2, GH 1, NO3 40, PO4 1 TDS 250