Watering with sand top layer

[Sammy]

I had an annoying fungus gnat problem. To get rid of them I implemented multiple methods I've read about including using a top layer of sand so the larvae can't get out, and the adults can't lay new eggs. However, how does one water their plant with the sand on top without it shifting and exposing the dirt underneath? Furthermore, how do you tell with sand when you need to water? Usually I can tell when the top inch or two of soil is dry, but that's not really possible with a layer of sand over top. Thanks for any advice!

 

[Chuck]

Stick your finger through the sand and water from the bottom up

 

[Sammy]

Stick your finger through the sand and water from the bottom up

That's an idea, thank you!