Actually it would take all of the military and government to be
sleeping for any country to launch a surprise attack on us.
Unless the launch 1 or 2 battalions only, which will be a pretty
futile effort, otherwise mobilizing multiple units - draw arms and
ammo, have transport unit start moving in our direction, have ships
manourvering to take better blockade or attack positions, numerous
aircraft loading and taking off, will definitely draw attention
from various monitoring units.
Radar, humit, signals, satellite and other assets would pick up
such movements pretty darn quick.