What you can see
In fog you only see tiles within your units' vision and around your buildings. Units in forests and reefs stay hidden unless one of your units is right next to them.
Infantry and Bazookas on a mountain see 3 tiles further. Rain cuts everyone's vision by 1. Scout Cars and SAM Trucks see far and are good spotters.
Traps
A unit that runs into a hidden enemy stops there. Move your strong units after a cheap scout has looked ahead.