Alright, this is how you know you choose omnivore: First, in every stage except space age you can click on a "your evolution history" in the bottom right corner. In the history at the far right it shows where you are according to carn/omni/herbivore. If you are leaning towards carnivore/aggressive, switch to herbivore/peacefull until you are in omnivore again. Check history often and make decisions accordingly and SAVE before you evolve, as the "evolve button" replaces history until evolution. If you are a carn/herbivore and not a omnivore after you evolve, load and play opposite for a bit and try again, however that does not work in tribal stage. Sorry if this post is too long.