I feel the best way to tell if a plant is thirsty is by sight.
Every strain will be different, and it also depends on your soil mix.
But generally, the plant is communicating its needs, in this 3 dimensional reality, with very simple cues.
Carefully study your plants every day, and soon you will be able to read them, and speak their language.
For example, study them after a heavy watering, study them after not watering for a day or two, study them when its hot, study them in the morning, evening etc...
Soon you will see, the plant is going through different states of being, subtly changing with the surrounding environment.
And when they are thirsty, hungry or anything else, they will tell you!
At least this is how I garden...