My daughters home has a small boiler to heat 2 rooms. There is no automatic device to add makeup water. There is a hand valve to add water. When should water be added and how do you know how much to add?

I've lived with two such boilers in my past, and both times the boiler had a glass sight gauge so you could see the water level in the tank. You add water when the level falls near the bottom of the gauge, and stop adding water when it gets near the top. Be careful not to overfill (ask me how I know)!