If the leader is clipping pro at their waist, the belayer will not notice much different.
If the leader is clipping pro overhead, then climbing past it, the belayer will notice that one rope in their belay device is tight and the other is slack, unless they manage the rope by feeding each rope through the device individually rather than both together. If you belay a lot on double ropes, you figure out how to do this (taking in/giving slack on two individual ropes) safely relatively quickly.