In some fundamental way we don’t really know, all you can do is to make a model and then test it as extensively as we can. Then, try to break it and improve it, making it more precise, more general etc. However several things makes that guessing easier
One thing is cosmological principle, ie assumption that laws of physics are basically the same everywhere. One argument that makes this assuption hold better is Noether’s theorem en.m.wikipedia.org/wiki/Noether's_theorem which means that at least some conservation laws are pretty fundamental. We don’t really know if laws of physics were the same in distant past, if they were then maybe there are some other, as of yet not discovered more fundamental laws of physics that hold in both conditions and simplify to what we know today in current conditions