The two pronunciations of 粘 粘 can be read as either "nian" or "zhan", but in both way it can be translated as "sticky", so what is the difference?
In other words what should I take in mind, when reading a text , in order to understand which pronunciation I should use?
Originally, you had two separate characters (zdic links):

粘 = zhan1 = (v.) to stick
黏 = nian2 = (adj.) sticky

The usage distinction still remains, but it is now acceptable to use 粘 for either word. Unihan lists the characters as semantic variants.