Regarding the signs before death shared on the internet such as the signs of 100 days before death and others. Are they true?
Death is included as one of the ghaibiyyat / sam’iyyat matters. Allah SWT emphasizes in al-Qur’an:
وَمَا تَدْرِي نَفْسٌ مَاذَا تَكْسِبُ غَدًا وَمَا تَدْرِي نَفْسٌ بِأَيِّ أَرْضٍ تَمُوتُ إِنَّ اللَّهَ عَلِيمٌ خَبِيرٌ
“And no soul perceives what it will earn tomorrow, and no soul perceives in what land it will die. Indeed, Allah is Knowing and Acquainted.” 
Allah SWT states:
وَعِنْدَهُ مَفَاتِحُ الْغَيْبِ لَا يَعْلَمُهَا إِلَّا هُوَ
“And with Him are the keys of the unseen; none knows them except Him.” 
If the signs are based on one’s personal experience, then they can be used as a general guide. However, one can’t firmly believe that they are the signs before death. However, what is certain is that the signs of 100 days before death spread on the internet is not based on any hadiths and it is prohibited to be said to be from the Messenger PBUH. Wallahua’lam.