When to really replace shoes?

I got fitted for some pretty pricey shoes with good insoles and all by a place that has you stand on the iPad etc. Been thrilled with them. That was a while ago now, maybe a year or even 2? I walk a lot, but have no idea how many miles I have on these shoes. Is there a way to tell from the shoes when I should replace them? I don't see any uneven wear or anything.