Well, if you are a hardcore programmer, probably Interface Builder on Mac or the integrated resource editor for Visual Studio can be good enough. But there are certain times to define how things work in advance, or those tools are not easy for GUI designers to use.
Then… it’s time you need a GUI prototyping tool.
There have been many, but nowadays, these two commercial S/W programs appear on Google.
We can use trial versions to see how they work.