How do I check wether a floating monster is touching walls, floors or ceilings?

[PeceMan]

I'm making custom monsters with decorate, and I'm trying to make sortof like projectile, pseudo lost soul, monsters, which die as soon as they collide with the environment.

The problem is that I don't know how to make the actor do something, or jump to a specific state, based on collision.

[Kronecker–Capelli]

What version of Decorate (Zardronum compatibility is desired)?
ACS solution is suited?
Why not use Zscript (made you mod Gzdoom/Lzdoom only compatible)?

Not tested example

actor missile_thing
{
monster
health 50
radius 20
height 20
  
states
{
  spawn:
    sprite frame tics a_chase();
  loop;

  melee:
  missile:
    tnt1 a 0 a_changeflag("missile", true)
    tnt1 a 0 a_changeflag("nogravity", true)
    tnt1 a 0 a_skullattack()// or how lost soul attack named??
  flying_loop:
    sprite frame tics //it should fly like a missile
  loop

  death:
    sprite frame tics
  stop
}

}