There are normally a couple things you can do to try to avoid this. If there are only two troop types listed in a province list of defenders, always ask yourself what the third may be. There are a limited number of available province defender groups so you can normally estimate what may be there. There is a list somewhere, I believe, but not in the mod inspector that I can tell as I initially thought. For example if you see slingers and no elephants (mammoths? forget the type) listed, they are likely there! Barring all that, many times, especially early game unless you are sure, send your scout in there with retreat orders to make sure. I believe many players do this whenever possible, and especially on thrones. Helps to find those hidden wizard defenders at times too (I still don’t know how to determine that otherwise)!
I am not trying to make light of it as it sucks when that happens and I hear you. It causes me to be much slower at expansion than what is optimal as I always fear losing too much of an initial expansion force in games. I hate to see people get crippled early on that causes them to lose enjoyment right away in a game. I am the same way. Good luck! Hopefully the province at least had a big income that helps jump start hiring more.