constant native IsUnitMasked takes unit whichUnit, player whichPlayer returns boolean
constant native IsUnitFogged takes unit whichUnit, player whichPlayer returns boolean

globals
  player owner = null
  group gr = CreateGroup()
endglobals

function Check takes nothing returns boolean
  if not (IsUnitFogged(GetFilterUnit(), owner) or IsUnitMasked(GetFilterUnit(), owner)) then
    // твои действия
  endif
  return false
endfunction

// . . .
  local unit u = null // наш юнит
  set owner = GetOwningPlayer(u)
  call GroupEnumUnitsInRange(gr, GetUnitX(u), GetUnitY(u), 500., Filter(function Check))
// . . .