How do I make sure my screen is locked?

if you mean locked when you wander off for a fresh cup of coffee,

System preferences > Security & Privacy > General ...

[√]. Require password after nnn minutes after sleep or screensaver begins

if momentary use by family members and so-called trusted associates are the issue, each should have their OWN Userid.

Users can log in and set your session aside if they need to "just look up one thing".

'Fast User Switching' is the way to 'Make it go"

Switch quickly between users on Mac - Apple Support

another option you have would be to set up a "hot corner" you can go into System Settings > Desktop & Dock and in the right panel, scroll to the bottom until you see the "hot corners" button. click the button.

and in the resulting window, set one of your corners to "Lock Screen". I use the bottom right corner for that purpose.

and then you can instantly lock your screen by moving your cursor to the corner you select.