Most houseplants can get the light and nutrients that they need on their own, but because it doesn't rain indoors, it is up to the plant owner to provide them with water.

An estimated 90% of houseplants are not watered correctly. They are either over watered and are subjected to root rot or other diseases or fungus, or they are not watered enough. It's really not that difficult to give your plants the correct amount of water to keep them healthy.

A simple hygrometer can be a great investment that will help you to water your plants correctly. More complicated ones can cost a lot, but simpler, yet still functional meters can be bought for under $10. Some of the hygrometers are digital and even contain thermometers to test soil temperature as well.

If you don't want to get this tool to measure water levels, it can be done manually.

By pressing your finger into the surface of the soil, you can tell if the soil is moist or dry. Obviously, if it's wet, it's very moist. If the soil gives in general, it is also moist. If it is stiff, it is dry. Similarly, if you press
a tongue depressor or chopstick into the soil and it comes out moist and dark, it shows that there is water under the surface.

Another method that you can use to measure how much water your plants are getting is to place a saucer under the pot and fill it with water until it is 1/3 full. Keep it there until it is absorbed up into the soil. This process should be repeated until there is water left in the dish, showing that the plant doesn't need any more water. You can soak up the excess water with a towel.  You should write down how much water was added before it stopped being absorbed, so you know how much water that plant and soil combination needs.

Once you know how much water your plant needs to be fully, but not overly hydrated, you can weigh the plant, (soil and pot included) just as it was watered the correct amount. As long as you don't take any soil out at any point, weighing each day will let you know if you are adding the right amount of water. You will get a feel for how much to water it after doing this for a while, and you will no longer need to weigh everything.

Keep in mind that house plants like the Zamioculcas need different amounts of water during different
seasons or if the air is hotter and dry or humid. As opposed to relying on one of the techniques above, using a combination of them will help to keep your plan correctly hydrated on a consistent basis.