Not that you need to be wasting time on this, but a good way to see where a crack is forming is to look at the bolts holding the sink to the wall. That's usually what causes a wall mounted sink to crack, either from age, stresses on the sink (i.e. a former tenant sitting on the sink), or just over-torquing of the bolts themselves. You should be able to see the cracks through the enamel. Unfortunately, there is no way to repair a cracked sink: you may have to completely forego the sink and use the tub faucet until they replace it.