Tick the checkbox next to the variable in the block palette. That creates a variable watcher on the stage. You can also right-click the watcher to change it to a large readout, displaying the variable contents within a minimal frame.

Every variable has a checkbox they makes it show on screen: https://en.scratch-wiki.info/wiki/File:Checkbox_to_show_or_h...

There is also a command block to show or hide it dynamically.

You could also display it with a say or think block.