Here are some pictures of a set of very worn hammers and some new ones. The worn ones are very grooved and you can see a hammer where the wooden molding is showing through where the felt is completely gone. The new hammers show a nice rounded shape.
What sound do worn hammers make? When played loudly the flat surface of the hammer imparts a square shape into the string and brings out a lot of high harmonics which make for a bright or brittle sound. When played softly the grooves can actually make the hammer stick on the string for a short moment and make it dull sounding.
Hammers can be resurfaced by filing off layers of the felt to restore a smooth striking surface and a rounded shape. However the hammers in this picture would not improve a lot from filing them because there is so little felt left. These hammers need to be replaced.