Write a function that takes in a list of numbers. The function should return a list of the indices of all numbers which are even.
For example:
>>> show_even_indices([2, 4, 6, 8])
[0, 1, 2, 3]
>>> show_even_indices([1, 2, 3, 4, 6, 8])
[1, 3, 4, 5]
>>> show_even_indices([7, 3, 11])
[]