Can I ask, how would you know when to do ghunnah for double kasra, double thumma and double fatha?

Question
Assalaamu alaykum wa rahmatullahi wa barakatuh,

Can I ask, how would you know when to do ghunnah for double kasra, double thumma and double fatha?

Answer
Wa alaikum assalaam wa rahmatullahi wa barakaatuh
The "double" fathah, dhammah, kasrah, is a symbol for the vowel plus an extra noon saakinah (non-vowled noon).  This is called a tanween.  The rules of noon as-saakinah and tanween help us determine how we pronounce -or don't pronounce- the noon saakinah or tanween depending on the letter which follows.  To learn the rules of the noon saakinah and tanween, please see the following link:
http://www.abouttajweed.com/the-noon-saakinah-and-tanween/index.php