The weapons you find will always indicate what you will fight up ahead:

WEAPON - WHAT IS AHEAD
Pistols - One pistol indicates Paranormal activity is nearby. Two pistols do not mean anything.
Sub machine - This indicates that there will be lots of any type of soldiers nearby.
Assault rifle - This does not means anything, as they are found throughout the game.
Nail gun - This indicates that long distance firing will be needed soon, or heavy armor is close by.
Shotgun - This does not usually mean anything, as they are found throughout the game. However, it may indicate heavy armor.
Rocket Launcher - Indicates that a Mech, Leviathan, or heavy armor is close by (not so much Heavy armor).
Repeating cannon - Indicates that lots of enemy soldiers, mechs, etc. are near. Basically, there is a lot of everything coming up. It also indicates that the end of the game is near when more quantities of them are found.
Particle weapon - Indicates possible snipers or heavy armor.
Mines - Indicates that a platoon of enemy soldiers is close by.