Out of the options, the best indicator is a color change since it is the only one that can't really be blamed on a physical change. you will eventually notice that during qualitative labs and some quantitative labs, usually the thing that you are looking for is either color change or the production of a precipitate to indicate the presence of a chemical reaction
It all depends what theory it is most are supported by really good evidence but they just don't have all the evidence so it can't be proven a fact at that time
