If the hard surface layer can be picked out from the joint, or if the mortar can easily be scraped out with, say, a door key, then this is a good indication that the wall may need repointing.
Os oes modd tynnu'r haenen galed o'r cymal neu os yw'n hawdd crafu'r morter gydag allwedd (er enghraifft), yna mae hynny'n arwydd bod angen ailbwyntio'r wal.