- Feel the leaves.
Touch leaves and give them a little squeeze. This works with many but not all plants...with a succulent, you can give the leaf a squeeze and feel when its really plump and full of water and when it has a lot of give and not so much water...with a fern..yeah no.
- Stick your finger in the soil (the most repeated answer ever)
Yes you can stick your finger sort of in the soil...but then your fingernail gets dirty. bleh. So try a wooden dowel or chopstick...stick that all the way down, pull it out, and feel the tip...is it moist/cool?
- Feel the weight of the pot.
Is it still on the heavy side? Notice how heavy it is after watering, that will give you a feel for the weight of the soil when fully saturated. Depending on the plant, you can also let it dry completely to a slight wilt (most plants can handle a little slight wilting...espeically if they know you'll take better care of them because of the helpful info you're getting) and notice the weight there. If you want to get fancy here, you can even mark your calendar from first watering to first wilt. Let's say it took 10 days to do that, then water once a week until conditions or plant-size changes.
- lift up the pot and look at the bottom
If you have big enough drainage holes in the bottom of the pot, you can pick it up and look at the soil in the bottom of the pot...feel it too...is it dark and moist?
- Actually look at the soil...is it dark or light?
Usually your typical potting soil gets lighter as it gets dryer.
- Look at the plant, is it droopy? (obvious? not always)
Some plants are total divas and droop intensely, some plants do not droop at all when they water, and just die. LOL So don't depend on this one completely.
- Notice the light levels.
Is the fern sitting on your south-facing-sun-blasted-pheonix-arizona high rise in the summer? It's gonna need more water. Is your cactus sitting on your north-facing-rain-soaked-english-patio-in-winter? Get it out of there! Too much cold and water not enough light!
- Notice wind levels.
This one is a sneaky one. I've had to water in Austin's "winter" even though light levels were low, because my community garden plot gets absolutely pummeled by wind. Wind is very drying.
- Notice humidity levels.
Want to put your African violet's growth and needs on hold without hurting the plant? Easy! put it in an aquarium or some other enclosed terrarium-type thing...they are specially adapted to take in water from the air via their totally cute little fuzzy leaves. I've put an African violet in a terrarium, forgot for 6 months, and it didn't even break a sweat...although I super don't recommend that for beginners. Two/three weeks though for a vacation it totally fine though. So if its very humid, you might not need to water.
- Notice soil texture.
Does you potting soil/garden soil seem thick and heavy and muddy? Probably don't need to water as often. Are you growing in gravel, clay pellets or sand or sandy-ish soil? More water please.
- Notice temperature.
Do you keep your house 80+ degrees? Well you'll need to water more often. Do you keep it your house closer to 45 degrees like my mother did all the time? Less water.
- Notice the type of plant
Snake plant? Less water. Venus fly trap? Have it sit in water all the time...preferably distilled, this diva will bite without her preferences met. (as a kid, I loved to sing with all the 90s divas...so I you'll see me turning plants into sassy divas in my imagination).
- Notice the root system
Is the plant filling the pot with roots? It will need more water more often. Does it have barely any root system? Careful with the water. Shallow small root system tend to (but ask me for you unique sitch) want small amounts of water, frequently that progressively get less frequent to encourage the root system to search for water so the root system gets more established. That's a lot of skill/work...so I usually a recommend covering the plant (unless its a cactus or something that doesn't like lots of water) to keep moisture levels naturally up while its getting a stronger root system.
- Notice plant stress levels.
If you're growing in Austin, you might have a wilty plant that doesn't need water. This is because when it gets really hot, the leaves can't keep up with the transpiration rate...basically water leaves the leaves (lol) faster than the roots can send up water. So the soil could be sopping wet, and the plant is still wilting. Your best bet here is a little shade and wait for the temperature to cool down.
- The size of the plant
Big plants tend to need large amounts of water, less frequently, where as small plants tend to need little amounts of water, more frequently, since they have less actual biomass holding water in general.
- The amount of airflow through the root system.
If you are growing an orchid on the side of a tree...you almost can't over water that thing. There is SO MUCH airflow around the roots that you can almost have water running over it all day. How about that same orchid in a pot of bark in a dark pot where not light or air moves through? Different story.
- The style of the root system...
Some plants store water in their roots. Think onions...those bulbs have so much water. So often but not always, you can get away with being a bit on the dry side with plants that store water.
- The watering style of the gardener...
Do you hover over each one of your plants like a helicopter dad? Checking twice a day for water? Well the plants will enjoy your worried carbon dioxide droplets getting all over them, but they won't enjoy as much water as you think. So hover away! But err on the side of less water.
- Or are you like Nancy?
My sister Nancy said "oh, I'm not so good with plants". I said really? Why's that? She said "well, I had a cactus once, but I think it died because...I never watered it" Umm what?! You'd have to go like a year without watering it! haha So if you're the opposite of a helicopter dad, then put some reminders everywhere to at least check for water once a week, or something like that.
- How often nature, or automated systems, is watering your plant.
Do you have a drip system? Do your lawn sprinklers accidentally water your potted plants too? Notice for any water that could be getting your plant outside...has it been raining a lot?
- I'm sure there are more factors, but by going through this list, you can get a really good feel for when your plant needs water.