hi jeremy,
is your wife due for an annual check up? if nothing is wrong ... is is the perfect time to get a base line.. a point of reference in case something were to develop (and as we age we have to expect the body to break down for time to time).
I am bipolar and am in a period of my illness' transition. So I have been educating my husband in the event he has to TAKE CHARGE. Either with love and concern or with harsh commands. If become suicidal/homicidal or psychotic he will need to take care of our young children first and DRAG me to a doctor second.
I am not telling you to drag your wife by the hair to a doctor... ;p but you might need to voice your concerns about her changes in mood and personality. You might use a soft voice of love and conern to convince her... you might have to use reason and proof ... you might have to be commanding and take charge.
But always do it with love and understanding. If your wife has depression she will automatically think the worst, think the negative, believe there is no hope, no help. Part of this programme teaches us how to reverse our negative habits of thinking. If your wife does not see the a problem you will have to be understanding, persuasive and wait.