Hey Growmie,
The best way to tell if a weed plant needs water is to stick a finger 1-2 inches into the soil if it’s wet, hold off, if it’s dry, it’s time to water. You can also pick up a pot and feel its weight to determine if it needs water. This will take some experience to be sure to lift up your pots after watering to get a feel for how heavy they are when full of water. This will also give you a sense of what a light, dry, plant feels like. This is what most growers use as a base for watering on average plants during the seedling stage need water every 4-7 days and every 2-4 days during the vegetative stage. A lot of this will also depend on the environment the plant is in also. Feel free to shoot me a holler if you ever need any help, I'd be honored to assist you. Hopefully, this was helpful, and remember ... "Happy Growing" and "Free your mind one puff at a time".