Use a correlation meter. If you are in the +1 area all of that will be mono compatible. Even if you have stereo information. All the elements that are below 0 will be out of phase and that is not the worst thing in the world. It just means that if someone would play back in mono that element would come in and out of the mix. If it's a pad or something absolutely in the back of the song I would not worry. Instead I would start listening to references and write down what elements have side information. Make a short list and apply to your own music. It's breath of fresh air once you go stereo.