Do you have "copy" checked?

Like any copy operation, you can't be really sure until you check the copied version. The easiest level of check is to see if the number imported matches the number you thought you imported. But agreement doesn't guarantee that the imports are without flaw-- you'd have to look at each one individually. But looking isn't good enough--that's just a first rendering. You would need to see if each one will open in Edit Mode to be sure.
That's a lot of trouble. That's why we keep at least two backups-- I have more, actually. For instance, I import pictures from my Nikon to Finder folders. I import pictures from the Finder folders to Photos. And I backup the Photos Library and the Finder folders to external drives. I don't delete pictures from my camera or from my Mac's Finder folders until I have edited, cropped, commented, and curated and backed up the pictures in Photos. I also have Time Machine backups.
But yes, you could delete the originals once they've been imported to Photos, if you're sure you have "Copy" checked in Photos' Settings.