I flew in an out of the ash layer several times today. It's only visible when you are level with it. But when you're in it you can smell it. So unless you can smell it's likely you're not in it. Also it had zero impact on the aircraft despite several climbs and descent through the layer which was quite thin where I was.
So for light aircraft no need to panic. Unless you plan to fly for an extended period in the ash layer. The stink alone will drive you out of it anyway. On the other hand I would tend to avoid any ash layer you could see from above or below. That might be nasty.