There are a number of ways for us to determine that Islam is the true religion. After one establishes the existence of God, they should automatically think of a way in how they can thank and get to know this God.
There are various worldviews and religions which claim to be true, but such a claim must be proven.
In my humble opinion, one should look at the Islamic concept of Tawhīd (the pure oneness and uniqueness of Allah). The Shia Islamic concept of God far surpasses all other religions and other Muslim sects that give Allah humanistic/material attributes. To know the true religion, one should start with the very foundation which is of the creator.
Shia Islam is the only religion which gives a pure monotheistic view of Allah being unlike His creation and completely unique. One should read the various narrations we have from the Imams (peace be upon them) about Tawhid.
Once you have a good grasp of Shia Tawhid, you will automatically realise that Islam is the true religion when you compare it with concepts such as the Christian trinity. Whilst the Islamic concept Tawhid is above the intellect, concepts such as the trinity, contradict the intellect. A religion cannot be true if it gives a view of God which contradicts the intellect.
This also applies to other religions such as Hindu’ism which focuses on worship of man-made gods that have been made into images. Images and idols are only made from the limited minds of man and the Prophet Mohammad (peace be upon him & his pure progeny) as well as other prophets, came to strictly emphasise on sole worship of Allah alone.
One should also take notice of how any current world view fails to provide a compete and perfect way of life which Islam can offer.
A final point would be to research into the linguistic miracle of the Quran
May Allah grant you success