There are 4 elements to be a valid contract, and one of them is an 'intention'(others are offer acceptance and consideration).

I know that there is an issue of intention when there is a contract between family members.

Sometimes, there is an intention or sometimes there isn't.

Then, how can we prove that there was an intention to create intention in a contract between family members.

For example, father promised his son to give a mobile phone if he passed all papers and his son accepted. 2 months later, the son showed that he passed all papers but his father refused it.

I can clearly see there is an offer and acceptance, and consideration(he sacrificed his time and effort to pass all papers). However, I cannot prove that there is an intention.

Can you explain this to me?

Thanks