TapNoiseKeyboard Viewer shows the characters assigned to your current Mac input source. It is useful when a symbol is printed differently on your physical keyboard or you cannot remember an Option combination.
Show the viewer
Open the Input menu in the menu bar and choose Show Keyboard Viewer. If the menu is absent, open System Settings › Keyboard › Text Input › Edit and enable Show Input menu in menu bar.
Hold Option, Shift or both and watch the displayed characters change. Try the character you want in a scratch document, then return to your work.
Check the active layout
The viewer follows the selected input source. Switching from one language layout to another can change punctuation and symbols even though the physical keycaps remain the same.
If your keyboard suddenly types a different symbol, look at the Input menu before buying a replacement keyboard. A mismatched layout is a software setting; a genuinely unresponsive physical key is a different problem.
Know the limits
The viewer is a guide to the selected layout, not a full shortcut list for the current app. Command-based actions are better checked in the app’s menus. Some accent combinations also involve a sequence rather than one simultaneous press.
For those, see typing accented letters. If you want to keep several languages available, use input source switching and check the menu whenever the output surprises you.
Source: Apple: use the Keyboard Viewer.