Is it obligatory to advise those who propagate evils on the internet, knowing that it is not known whether they are Muslims or not?

It is impermissible to remain on a page that contains evil (munkar), and it is obligatory to denounce and change it, subject to certain conditions. If it is highly probable that they are disbelievers, then denouncing them may or may not be obligatory. Scholars have clarified that what must be denounced for a disbeliever is that which is forbidden to Muslims and causes them harm. If they reside among Muslims, they are held accountable for what Muslims are held accountable for. They are not to be interfered with regarding what they do not openly display and believe to be permissible in their religion, as long as it does not harm Muslims. However, if they openly display evil, it must be denounced. A dhimmi (a non-Muslim living under Muslim rule) is prevented from what a Muslim is prevented from, except for what is explicitly exempted in their covenants, such as drinking alcohol and eating pork. If a dhimmi openly commits forbidden acts according to Islamic law, they are to be reprimanded. If it is possible to report the matter to the authorities, then it is incumbent upon you to do so, and there is no objection to advising them.