Even though California voters decided that they don't want labeling of food sourced from genetically modified organisms, you can still tell. Just learn the codes on the standard produce stickers and choose wisely.
A 5 digit code starting with 8 is GMO food - avoid. A 4 digit code is conventionally grown produce - keep moving. A 5 digit code starting with 9 is organic produce - bingo! It's as easy as that. I may just learn to love those stickers after all. (I have a long standing hatred of stickers on food and in my compost.)
Better still, buy produce at the local Farmers Market and know your vendors, or grow some veggies and fruit in your own (organic) garden. But the world is not always and everywhere this perfect, so learn those codes.