Quote Originally Posted by blazer-glory
One thing that crossed my mind was how can you tell if your defeat was down to lack of numbers and not bad tatics?
You cannot. The difference between victory and defeat can depend on so many things, like terrain, weather, troop types, numbers, tactics and the number of command stars the general has, that it is often hard to say whether a battle is winnable. It takes experience to determine which odds you can take-on under given conditions, and as your experience grows you will be able to win against worser odds.

So the answer to your question is: experience, but that is not a helpfull one. When I started Shogun, I needed a big hill to gain a decisive advantage. When I had played it for half a year, I could trash armies almost twice my size (and I don't consider myself a good player).