Pictures folder in Finder and Photos

Open your library and create a smart album with these criteria:

If you get zero items matched you're running a totally managed library. That means that you can delete the corresponding source files from your Pictures folder. What is checked in your library's General preference pane?

iCloud will only sync with photos that are in your library. If you keep another set in the Pictures folder you might want to move them to an external hard drive as a second backup.

You don't need to and want to as you would be doubling the storage requirements on your hard drive. Once you import photos into your library and you verify the import was successful you can delete the source file from your hard drive. This, of course, assumes you're running a "managed" library:

i.e. you're copying item into the library during import. It's not recommended to use a "managed" library.

Don't delete the Pictures folder but the contents, i.e. sub folders of image files. Do you have a backup strategy in place, i.e. like Time Machine or similar?

If not I'd start one before doing any major deletions.

Import the images in your Pictures Folder to Photos. It will flag duplicates along the way.

In Photos: File > Import...