2. Previewing your HUD in Unity - SuedOnAIM/RF_CustomHUD_Edits GitHub Wiki
The Game Window
You can preview your HUD without going ingame by using the "Game" window.
Using the Game window only lets you see how the HUD looks like and doesn't run any scripts. So if you have dynamic features, like a kill feed, you can only check if it works by exporting and testing ingame.
Make sure you have a Game window. If you don't have one, you can add it by clicking on the three vertical dots then adding a new game tab.
Your HUD needs to be in your scene's hierarchy. If it's not, you can drag your HUD prefab from the project files into the hierarchy window.
When you click on the game window, your HUD should display like this.
I recommend changing your aspect ratio to be 16:9 since that is the most common aspect ratio for monitors.