body_enter
signal will tell you about each collision when they happen (you need contact monitor).
get_colliding_bodies
gives you an array with the bodies colliding IF contact monitor is true (default false), the size of the array will be as large as the collision max count (default 0).
Kinematic bodies works different, these have a simple collision detection that works only when (after) using move
or move_to
during _fixed_process
Kinematic character and platformer demo have many examples.