The things I use are part math, part visual interpretation, with the help of some guides if I get stuck.
It can be as easy as looking at the stats and certain situations, or as complicated as finding travel times across arena length, slow motion interactions or calculating how much damage a specific card is likely to do. None of this is tested on private servers as I do not have access to them.
If you wanted to learn more about it @Giorgakis
, I've left some details in my Blimp thread, and may put some here as well.