How can one know that he has perfectly rendered gratitude for a blessing, and does the promised increase in His saying, the Almighty: "If you are grateful, I will surely increase you [in favor]" have signs, and does it encompass this world and the Hereafter?

The sign of gratitude is the feeling of inadequacy in thanking Allah, and that success in giving thanks is a blessing that necessitates further thanks. Gratitude has levels, the highest of which is acknowledging one's inability to fully express it. Gratitude is achieved by recognizing blessings from Allah with the heart, using one's limbs in obedience to Him, and praising Allah with the tongue. Among the means of gratitude is remembering Allah frequently, which is the essence of gratitude, for whoever does not remember Him does not thank Him. Gratitude is a cause for the increase of blessings in this world and reward in the hereafter, and its reality is acknowledging the blessing to the Bestower, and not using the blessings in disobedience to Him.