Molecular biologists aren't stupid. They fluorescent tagged the pieces that get incorporated into the DNA and not into anything else. There will have been positive and negative controls to gain confidence that the tagging worked as expected.

You might want to lose the snarky attitude as it's not a good look when you're really ignorant about the techniques you're sneering at. You seem curious. Just learn without the snark.

EDIT> This is the kind of thing you learn in 1st and 2nd year university biology courses. Especially cell bio / molecular bio lab technique courses.