There is something called the sound Fitrah (natural disposition) or the sound logic. So, whatever conforms to logic and to the sound natural disposition and the sound mind is from Allah, and whatever is complicated is from human beings.
For example,
If we were told by a cleric, whether a Muslim, Christian, Hindu, or of any other religion, that the universe has one Creator alone, Who has no partner and no child, and Who does not come to earth in the form of a man, an animal, a rock, or an idol, and that we must worship Him alone and resort to Him alone during calamities, then verily, this is the religion of Allah. However, if a cleric, be he a Muslim, Christian, Hindu, etc., tells us that Allah is embodied in any form that is known to mankind and that we must worship Allah and resort to Him through some person, prophet, priest, or saint, then this is from human beings.
The religion of Allah is clear and logical and free from mysteries. So, if a cleric wants to convince someone that Muhammad (may Allah's peace and blessings be upon him) is a god and we must worship him, he will have to exert a great effort to convince him and he will never be convinced because he might ask: How could Prophet Muhammad be god despite the fact that he used to eat and drink like the rest of us? The cleric may even end up saying to him: You are not convinced because this is a mystery and an ambiguous concept, but you shall understand it when you meet Allah. This is what many do today in justifying their worship of Jesus, Buddha and others. Such an example proves that the right religion of Allah must be free from riddles, which come only from human beings.
Moreover, the religion of Allah is free, i.e., everyone has the freedom to pray and worship Allah in His houses without having to pay fees to get a membership and be able to practice worship there. However, if people are forced to register and pay money in any place of worship to practice worship, then this is from human beings. It must be noted that if a cleric tells people that they have to give out charity to help people directly, then this is from the religion of Allah.
People are equal in the religion of Allah. There is no difference between an Arab and a non-Arab, a white and a black except in terms of piety. So, if some believe that there is a certain mosque, church, or temple for white people only, whereas black people have a separate place, then this is from human beings.
Honoring woman and holding her in high esteem, for instance, is a command from Allah; yet, suppressing women is from human beings. Thus, if there is a suppressed Muslim woman in a certain country, for example, you shall find suppressed Hindu, Buddhist, and Christian women in the same country, as this is the culture of nations that has nothing to do with the right religion of Allah.
The right religion of Allah is in a constant conformity and harmony with the natural disposition. For example, anyone who smokes cigarettes or consumes alcohol will always ask his children to keep away from smoking and consuming alcohol because deep down he knows how perilous they are for health and society. So, when the religion prohibits intoxicants, for instance, this is indeed one of the commands of Allah. However, the prohibition of milk makes no sense because everyone knows how useful it is for health, that is why it was not prohibited by the religion. Allah, out of His mercy and kindness, has permitted us to eat what is good and prohibited us from eating what is evil.
The head-cover for woman and the modest clothes for both men and women are, for instance, from the commands of Allah; however, the details concerning the colors and designs are from human beings. The atheist Chinese woman and the Christin Swiss female farmer wear a head-cover based on the fact that dressing modestly is part of the natural disposition.
Terrorism, for example, is widespread in various forms around the world among sects from all religions. There are Christian sects in Africa and all around the world that kill and practice the worst forms of suppression and violence under the name of religion and Allah. They represent 4% of the Christians in the world. While those who practice terrorism under the name of Islam represent only 0.01% of the Muslims in the world. Not only so, but terrorism is widespread as well among the Buddhist and Hindu sects and other sects from other religions.
This is the way we can distinguish between the truth and falsehood before reading any religious book.