How can one guarantee the sincerity of intention for God in good deeds such as giving gifts and visiting, while avoiding their intermingling with the desire for people's affection or obtaining worldly benefit from them?

The first is that the act should be purely for the sake of Allah the Almighty. However, there is no harm in giving a gift with the intention of Allah's pleasure and love for the recipient. Aspiring to the fruits of deeds does not contradict sincerity if the intentions are aligned with the purposes of the Lawgiver.