My plumeria droop when they have too much water and when they are too dry - go figure! Usually though, I can tell by sticking my finger in the soil - if I stick my finger in and I'm even "not sure" that generally means too much water. I wait until mine are bone dry to water, and as a result I sometimes water only once or twice a year!
To be fair - I don't count rain as me watering, and it rains enough that I don't worry too much. I am getting to the point where I can look at my plumeria from 50 feet away and know their "mood" from the leaves.
