Citrus black spot is a fungal disease caused by Guignardia citricarpa. A way to know for sure is normally the leaves and the rest of the tree will be symptomless and have almost no marks.
You need to watch your watering during the rainy season and avoid allowing the soil to be too wet for extended times. Keep the top 2 inchs of soil loose and aerated. Also pick up your leaf litter every winter and through out the year, because the fungal spores live on the leaves and reinfect the tree every year.
Excess fertilizer can also add to your problems and cause the Citrus Black Spot to spread like crazy.
