You can see the wear on the cogs if you have an unworn one to compare them to. An unworn cog will have almost perfectly circular "valleys" between the teeth. On a worn cog, the "valleys" will clearly look oval-shaped when next to a new, unworn cog.

I find it pretty hard to see without an unworn and preferable identical cog/cassette handy, but it jumps right out at me when I do have both the worn and unworn ones.