test to differentiate between NaCl and zinc hydroxide
You can use the large difference in water solubility to determine whether the residue is sodium chloride or zinc hydroxide. Sodium chloride is very soluble in water and zinc hydroxide is insoluble in water. You can do the test as follows:  

1) Brush or scrape a portion of the residue onto a piece of paper or card stock.
2) Transfer the residue onto a dark colored plate or bowl.
3) Add enough water to cover up the residue and swirl gently. 

Any white solid that remains should be zinc hydroxide as the sodium chloride will dissolve away.