You know when you are feeling the sweetness of iman when your hearts at rest and you feel contentment, when you don't struggle to perform ibadah and you detach yourself from dunyah.
May Allah make it easy for us
oh another thing, we do acts of sincerity in sha Allah - read up on repentance, I think one of the signs of acceptance of repentance is that you perform more good deeds, that's what I call sweetness of iman. 8)
و عليكم السلام و رحمة الله