Five steps to your perfect mattress - support and alignment

Hello,
I am curious if there is a recommendation for shoulder alignment when testing for alignment while laying on your back. In this post, you talk about mid/upper back pain caused by a [quote]mattress that has comfort layers that are too thick and/or soft and allows your torso to sink in a little too far but “holds up” the lighter shoulders (pushing them forward).[/quote]
Is there a way to test to make sure this isn’t happening or alignment that I should be looking for across the shoulders when laying on my back? It seems too simple to just say that my shoulders should be in a straight line with no curve, but is that the case?
Thanks!