Conventional produce is labeled with a 4-digit number, such as #4166 on the onion I've shown. Organic produce is prefixed with the number 9. If this onion was organic, its PLU would read #94166. Genetically modified (GM) produce is prefixed with the number 8, e.g. #84166 for a GM onion.
Growers aren't required to label GM produce as such. There was a time they thought people would pay more for it, but lately, they've decided not to divulge. If you do see any 8-prefixed fruits or vegetables, I'd love for you to note them in comments. I haven't seen any.
However! If produce is prefixed with a 9, for organic, you can be sure it wasn't genetically modified.
Just for fun, here's a site where you can enter the PLU of any item of produce and receive back its data. Try it! (A few innocuous questions will get you through to the site.)
International Federation for Produce Coding