Always try to see things through her eyes. NEVER dismiss her depression as mere sadness or attribute her behavior to having blues; depression is different beast altogether. Sit her down, and discern what are the things that bother her, then try to look for ways to address each of them. Depression is a mental disorder, and in its severe cases, medical help may be needed.
Try to be more affectionate and empathize with her situation. It's always best to talk it out and let her know that you are always on her side. You don't need to bluntly ask if she's depressed. Start with small talks. Ask her if she's okay, how is her day and how you could help her make her day better. Depression is real especially for SAHMs. I hope this helps :)