The operator should look all around and check properly over both shoulders before reversing out of the stack. The safest approach is to look behind last thing i.e. engage reverse and release the parking brake first whilst keeping the service brake applied. After looking behind, check that the forks are coming out of the stack without fouling anything and then continue to look behind with occasional glances at the forks.
Some trucks are fitted with mirrors. Whilst there is no harm in using them as an additional check they should not replace a physical look behind.