Actually, we can detect design, and also detect those things that have not been designed.
The latter is the more important point. It is absolutely clear that living things are not designed. It is absolutely clear that rocks, sand dunes, tree rings, galaxies, mountains, are not designed.
The issue is the utter nonsense called "Intelligent Design".
There are people that make claims that there is some universal designer, but they have never presented any evidence of that "designer" much less that that designer is in any way intelligent.
Anyone so limited that they can only spell a word one way is severely handicapped!